What does this PR do?

Moves packages/agents off the archived utopia-php/fetch onto utopia-php/client, which the rest of the codebase already uses.

The problem

Every agent call built a new fetch client. Its cURL adapter registers non-static closures on the handle, and a closure declared in a method captures $this, so each call left a reference cycle that refcounting can never free:

  per call:

  CurlHandle ──> closure ──> Adapter ──> $this->handle ──> CurlHandle
      ^                                                        │
      └────────────────────────────────────────────────────────┘
                     cycle: only the GC can free it

  call 1   [handle + TLS + socket]   ─┐
  call 2   [handle + TLS + socket]    ├─ pile up until gc_collect_cycles()
  call 3   [handle + TLS + socket]    │  (rarely reached in a long-lived worker)
  ...                                ─┘

On Cloud the threats worker grew about 50 MB an hour, accumulated CLOSE_WAIT sockets to api.openai.com, and restarted at its 1 GiB limit. utopia-php/fetch is archived, so the cycle cannot be fixed there.

The fix

Each adapter builds one client on first use and sends every request through it. Inside a Swoole coroutine that is a pool of keep-alive coroutine clients; elsewhere a single keep-alive cURL client that uses static closures and one reset handle:

  before                              after

  send() ─> new Client ─> leaked      send() ─┐
  send() ─> new Client ─> leaked      send() ─┼─> one client ─> keep-alive connection(s)
  send() ─> new Client ─> leaked      send() ─┘   (pool of 8 inside Swoole)

• Every adapter constructor takes an optional $client, for processes that already share a pool. There is no setter.
• setTimeout() rebuilds a client the adapter built and leaves an injected one alone.
• A request waits for a free pooled connection as long as it would wait for a response, so a busy pool delays a call instead of failing it.
• Streaming goes through the PSR-18 stream() sink, so SSE processing takes raw string chunks.

This is a breaking release of the package: PHP 8.5 (matching utopia-php/client), string chunks instead of Utopia\Fetch\Chunk in process(), and the OpenAI::createClient() hook replaced by headers(). Root composer.json declares psr/http-client, which it already hoisted.

Test Plan

• bin/monorepo check agents and bin/monorepo test agents pass (139 tests). New tests cover the injected client, reuse across sends, the setTimeout() rebuild rules, request shape, SSE reassembly across chunk boundaries, and error status propagation.
• Reproduction in appwrite/ce:cl-2.2.0-1 (Swoole 6.2.2, PHP 8.5.10) running the real Agent/OpenAI/Conversation code with GC disabled: before, 200 calls grew RSS by 18 MB and leaked 2 fds per call; after, RSS grows 2.5 MB in total and the fd count stays flat.

Related PRs and Issues

Cloud consumes appwrite/server-ce at dev-main, so composer update appwrite/server-ce there picks this up for worker-threats once merged.

Checklist

• Have you read the Contributing Guidelines on issues?
• Have you added tests?
• Do all tests pass locally?