fix(openai): discard stale pooled Responses WebSocket connections

[JSap0914]

Summary

Fixes #6513.

The OpenAI Responses WebSocket transport (openai.responses.LLM(use_websocket=True)) pools connections through ConnectionPool. ConnectionPool.get() only validated max_session_duration before returning an available connection — it never checked whether the connection was still open.

After concurrent requests grow the pool (a single Responses socket handles one in-flight response, so parallel work creates multiple sockets) and those sockets sit idle, a peer / NAT gateway / VPN / proxy can close them. _ResponsesWebsocket.generate_response() then acquires a dead socket and fails at ws.send_str():

failed to send request over WebSocket, retrying in 0.1s
failed to send request over WebSocket, retrying in 2.0s

Each stale pooled connection burns one outer LLM retry attempt (and its backoff) before a healthy connection is reached, adding noticeable latency to a live voice turn.

Change

• Add an optional validate_cb: Callable[[T], bool] | None to ConnectionPool. It is invoked before a pooled connection is handed out; connections that fail validation are removed/closed and get() keeps searching (or creates a fresh connection).
• Wire the Responses pool with validate_cb=lambda ws: not ws.closed, so a socket that was closed while idle is discarded before it reaches the caller instead of surfacing a send failure and retry.
• The existing retry behavior is preserved for connections that become unusable during the actual send (a live race), which validation cannot catch.

This follows the minimal approach suggested in the issue.

Tests

tests/test_connection_pool.py:

• test_get_discards_invalid_connection — a healthy connection is returned even when stale (closed) connections are present in the pool.
• test_get_creates_new_when_all_invalid — when every pooled connection fails validation, get() discards them and creates a fresh connection.
• test_get_reuses_when_no_validate_cb — behavior is unchanged when no validate_cb is configured.

$ python -m pytest tests/test_connection_pool.py -q
7 passed

ruff format --check and ruff check pass on the changed files; mypy clean on connection_pool.py.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.