Moves chat model override settings from deployment scope to organization
scope, following the org-scoping of chat model configs in #28440.

Depends on #28440 (based on
`ethan/chat-models-availability-consolidation`).

Remote dogfood UAT passed on this change, including multi-organization
scenarios on a licensed deployment with real models.

## Problem

Chat model configs are now owned by organizations, but the override
settings that reference them stayed deployment-wide: admin
subagent/title/compaction overrides in `site_configs`, personal
overrides in `user_configs`, and the advisor model embedded in the
advisor runtime JSON. A single deployment-level override can only point
at one organization's model config, so overrides effectively worked for
the default organization only, and title generation had to soften its
hard-failure contract to tolerate unusable overrides.

## Fix

Admin overrides (general, explore, title generation, compaction,
advisor) and personal overrides (root, general, explore) become
organization-scoped rows in two new typed tables,
`chat_organization_model_overrides` and `chat_user_model_overrides`.
Composite foreign keys on `(organization_id, model_config_id)` make
cross-organization model references unrepresentable. The migration
deletes the legacy serialized `site_configs`/`user_configs` override
keys instead of migrating them; admins and users re-select models in the
new organization-scoped settings.

New endpoints live under
`/api/experimental/organizations/{organization}/chats/model-overrides`
(admin defaults) and
`/api/experimental/organizations/{organization}/members/{user}/chats/model-overrides`
(personal overrides); the legacy deployment-level routes are removed.
chatd resolves overrides via the chat's organization, restoring the
hard-failure contract for configured-but-unusable title generation
overrides. The deployment-level `AllowUsers` switch and advisor runtime
limits stay deployment-scoped.

The Coder Agents settings UI gains a per-organization "Defaults &
overrides" page under AI Settings > Models, and the personal overrides
page gains an organization selector for users in multiple organizations.
Advisor telemetry now reports per-organization override rows.

**BREAKING CHANGE**: Existing deployment-level overrides are dropped,
not migrated. Every organization starts with no overrides after upgrade,
and model selections must be re-applied in the new settings. The legacy
deployment-level override endpoints are removed.

Closes https://linear.app/coder/issue/CODAGT-872

> [!NOTE]
> Xum (AI agent) authored this PR on behalf of @ibetitsmike.

<!-- xum-attribution: model=claude-opus-4-6 thinking=high -->

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

## Summary

Consolidate chat model management and runtime availability into a single
organization model collection contract. The organization model response
now includes provider availability, unavailable reasons, and
unsupported-provider details, so callers no longer need a separate
availability request.

Depends on #28439.

## Problem

The organization-scoped API exposes two overlapping representations of
the same chat models:

```text
GET /api/experimental/organizations/{organization}/chats/models
GET /api/experimental/organizations/{organization}/chats/models/available
```

The collection route returns organization-owned model configurations and
redacted provider descriptors. The availability route returns a second
catalog-oriented response with provider availability and synthetic
catalog entries. Callers must fetch, reconcile, and cache both responses
even though they describe the same effective set of models.

This creates duplicated server logic and separate SDK, OpenAPI, and
frontend types. It can also produce inconsistent client state when one
request succeeds while the other fails or when the two responses are
refreshed at different times.

## Fix

Return all model-management and runtime-availability information from
the organization model collection route:

```text
GET /api/experimental/organizations/{organization}/chats/models
```

`OrganizationChatModelsResponse` continues to return the caller-readable
organization model configurations and redacted provider descriptors.
Provider descriptors now also include:

- `available`
- `unavailable_reason`

The response additionally includes `unsupported_providers`, allowing
clients to explain configurations that the Agents harness cannot use.

Frontend model settings, agent creation, existing chat recovery, and
model override surfaces now consume this single collection query. This
keeps model ownership, caller visibility, provider availability, and
unsupported-provider guidance in one cache entry and one authorization
path.

The default-organization compatibility route remains available:

```text
GET /api/experimental/chats/models
```

It now returns the same consolidated `OrganizationChatModelsResponse`
contract instead of the former availability response.

## Breaking change

This removes the separate organization model availability endpoint:

```text
GET /api/experimental/organizations/{organization}/chats/models/available
```

Clients using that endpoint must migrate to:

```text
GET /api/experimental/organizations/{organization}/chats/models
```

This PR also removes the corresponding public experimental client and
schema surface:

- `ExperimentalClient.ChatModelAvailability`
- `ChatModelAvailabilityResponse`
- `ChatModelProvider`
- `ChatModelCatalogEntry`
- The generated OpenAPI schema and frontend API/query types for the
availability endpoint

Callers must use `ExperimentalClient.ChatModels` and
`OrganizationChatModelsResponse` instead. Availability is now reported
on each `ChatModelProviderDescriptor`, and unsupported providers are
returned in `OrganizationChatModelsResponse.UnsupportedProviders`.

The default-organization compatibility route retains its URL, but its
response schema changes from `ChatModelAvailabilityResponse` to
`OrganizationChatModelsResponse`. Direct HTTP clients and generated
clients that decode the previous response type must be updated even if
they do not use the removed organization-scoped `/available` route.

Moves chat model override settings from deployment scope to organization
scope, following the org-scoping of chat model configs in #28440.

Depends on #28440 (based on
`ethan/chat-models-availability-consolidation`).

Remote dogfood UAT passed on this change, including multi-organization
scenarios on a licensed deployment with real models.

## Problem

Chat model configs are now owned by organizations, but the override
settings that reference them stayed deployment-wide: admin
subagent/title/compaction overrides in `site_configs`, personal
overrides in `user_configs`, and the advisor model embedded in the
advisor runtime JSON. A single deployment-level override can only point
at one organization's model config, so overrides effectively worked for
the default organization only, and title generation had to soften its
hard-failure contract to tolerate unusable overrides.

## Fix

Admin overrides (general, explore, title generation, compaction,
advisor) and personal overrides (root, general, explore) become
organization-scoped rows in two new typed tables,
`chat_organization_model_overrides` and `chat_user_model_overrides`.
Composite foreign keys on `(organization_id, model_config_id)` make
cross-organization model references unrepresentable. The migration
deletes the legacy serialized `site_configs`/`user_configs` override
keys instead of migrating them; admins and users re-select models in the
new organization-scoped settings.

New endpoints live under
`/api/experimental/organizations/{organization}/chats/model-overrides`
(admin defaults) and
`/api/experimental/organizations/{organization}/members/{user}/chats/model-overrides`
(personal overrides); the legacy deployment-level routes are removed.
chatd resolves overrides via the chat's organization, restoring the
hard-failure contract for configured-but-unusable title generation
overrides. The deployment-level `AllowUsers` switch and advisor runtime
limits stay deployment-scoped.

The Coder Agents settings UI gains a per-organization "Defaults &
overrides" page under AI Settings > Models, and the personal overrides
page gains an organization selector for users in multiple organizations.
Advisor telemetry now reports per-organization override rows.

**BREAKING CHANGE**: Existing deployment-level overrides are dropped,
not migrated. Every organization starts with no overrides after upgrade,
and model selections must be re-applied in the new settings. The legacy
deployment-level override endpoints are removed.

Closes https://linear.app/coder/issue/CODAGT-872

> [!NOTE]
> Xum (AI agent) authored this PR on behalf of @ibetitsmike.

<!-- xum-attribution: model=claude-opus-4-6 thinking=high -->

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

> [!IMPORTANT]
> Most of the added lines in this PR are generated API reference content
for publishing the organization-scoped `GET` and `POST
/api/v2/organizations/{organization}/chats/models` replacements. There
is no matching generated-doc deletion because the three removed
default-organization endpoints are experimental and are not present in
the generated public API reference on the current base. The removed
paths remain only as explicit 404 reservations, with no
default-organization shim or functional handler.

## Summary

Remove the unused default-organization chat model collection routes:

```text
GET  /api/experimental/chats/models
GET  /api/experimental/chats/model-configs
POST /api/experimental/chats/model-configs
```

Use the organization-scoped collection instead:

```text
GET  /api/v2/organizations/{organization}/chats/models
POST /api/v2/organizations/{organization}/chats/models
```

## Context

The intention of PR #28440 was to consolidate model availability into
the organization-scoped models collection for CODAGT-898 and remove the
superseded collection routes. That PR removed
`/organizations/{organization}/chats/models/available`, but it missed
these older default-organization routes and explicitly retained one of
them. This PR completes the intended #28440 cutover. The repository has
no SDK, CLI, or frontend consumer for the removed routes.

This change also prevents PR #28496 from promoting the missed
default-organization `/chats/models` route into the stable `/api/v2`
API.

This change is separate from the one-release `/api/experimental`
compatibility window in CODAGT-921. That window preserves experimental
versions of the intended stable API. It does not require compatibility
routes for unused deployment-scoped endpoints.

Refs CODAGT-898.

## Breaking change

Clients that call the removed routes must use the organization-scoped
model collection and provide an organization.

## Changelog

Remove unused default-organization Coder Agents model API routes. API
clients must use the organization-scoped chat models collection.

> [!NOTE]
> Coder Agents generated this pull request on Ethan Dickson's behalf.

> [!IMPORTANT]
> Most of the added lines in this PR are generated API reference content
for publishing the organization-scoped `GET` and `POST
/api/v2/organizations/{organization}/chats/models` replacements. There
is no matching generated-doc deletion because the three removed
default-organization endpoints are experimental and are not present in
the generated public API reference on the current base. The removed
paths remain only as explicit 404 reservations, with no
default-organization shim or functional handler.

## Summary

Remove the unused default-organization chat model collection routes:

```text
GET  /api/experimental/chats/models
GET  /api/experimental/chats/model-configs
POST /api/experimental/chats/model-configs
```

Use the organization-scoped collection instead:

```text
GET  /api/v2/organizations/{organization}/chats/models
POST /api/v2/organizations/{organization}/chats/models
```

## Context

The intention of PR #28440 was to consolidate model availability into
the organization-scoped models collection for CODAGT-898 and remove the
superseded collection routes. That PR removed
`/organizations/{organization}/chats/models/available`, but it missed
these older default-organization routes and explicitly retained one of
them. This PR completes the intended #28440 cutover. The repository has
no SDK, CLI, or frontend consumer for the removed routes.

This change also prevents PR #28496 from promoting the missed
default-organization `/chats/models` route into the stable `/api/v2`
API.

This change is separate from the one-release `/api/experimental`
compatibility window in CODAGT-921. That window preserves experimental
versions of the intended stable API. It does not require compatibility
routes for unused deployment-scoped endpoints.

Refs CODAGT-898.

## Breaking change

Clients that call the removed routes must use the organization-scoped
model collection and provide an organization.

## Changelog

Remove unused default-organization Coder Agents model API routes. API
clients must use the organization-scoped chat models collection.

> [!NOTE]
> Coder Agents generated this pull request on Ethan Dickson's behalf.

(cherry picked from commit 0f5c7b3)