… sum (#27967)

## Problem

CODAGT-906: chats using OpenAI-compatible backends (e.g. poolside)
through the AI Bridge persist token usage inflated 105x-640x, which
falsely triggers automatic chat compaction on every turn.

The chat-completions streaming interceptor summed usage across every SSE
chunk of one upstream stream and rewrote each relayed usage-bearing
chunk with that running sum. Spec-compliant OpenAI emits usage once
(final chunk with `stream_options.include_usage`), so the sum equals the
final value. vLLM-style backends emit cumulative usage snapshots on
every chunk, so the relayed final usage becomes roughly `N_chunks x
prompt_tokens` (e.g. 417,012 persisted for a ~6,000-token context).
chatd persists that value per assistant message and its compaction
trigger reads it as context occupancy.

## Fix

Track the latest usage-bearing chunk's raw usage (last-wins) in the
stream processor, updating only when a chunk actually carries usage so a
trailing usage-less chunk cannot zero it. `marshalChunk` relays that
value and `recordTokenUsage` records the same value, unifying relayed
and recorded usage. Last-wins is correct for both shapes: a single final
usage chunk, and cumulative snapshots where each snapshot already
includes all prior tokens.

Per-iteration semantics are unchanged: each tool-loop iteration has its
own processor, and the final iteration's usage is what the client sees.

## Tests

- `TestStreamProcessorUsage` (internal): cumulative snapshots with a
trailing usage-less chunk, and the spec-compliant final-only shape;
asserts relayed and recorded usage equal the last snapshot.
- New txtar fixture `streaming_cumulative_usage_injected_tool.txtar`
with per-chunk cumulative usage plus an injected tool call; asserts
client-visible final usage through the full interceptor.
- Red-green verified: with the fix reverted, the internal test reports
zeroed usage (trailing chunk overwrite) and the fixture test reports
18000 summed prompt tokens instead of 6000.

The blocking (non-streaming) path deliberately keeps its cross-iteration
summation for external clients and is untouched. Remote dogfood UAT
validated chat streaming, tool calls, plausible usage numbers, and zero
spurious compactions.

> Mux acted on Mike's behalf to author this change.