fix(proxy): surface codex websocket loop failures in livez

[rodboev]

Description

Codex /v1/responses WebSocket disconnects can trigger a known websockets callback failure before connection_made() initializes recv_messages. When that happens, the proxy process can stay alive while /livez keeps advertising a clean healthy state. This change contains that known callback failure in the proxy runtime, records loop callback health, and makes /livez report the degraded state instead of always returning a clean process-alive payload. Closes #1720

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• Add a proxy-owned asyncio loop exception handler that recognizes the known websockets connection_lost ClientConnection.recv_messages AttributeError, records it in bounded runtime health state, and leaves unrelated loop exceptions delegated to the previous or default handler.
• Extend /livez so the route remains cheap and unauthenticated while reflecting recorded event-loop callback health instead of always reporting a clean process-alive payload.
• Preserve existing Codex WebSocket relay, fallback, session deregistration, and termination-cause behavior for normal handler-owned failures.
• Add focused regression coverage for the known callback failure, the negative-space delegation path, and the health route response after loop callback degradation.

Testing

• Unit tests pass (uv run pytest tests/test_proxy_healthchecks.py tests/test_openai_codex_ws_lifecycle.py tests/test_proxy_loop_exception_health.py -q)
• Linting passes (uv run ruff check headroom/proxy/server.py tests/test_proxy_healthchecks.py tests/test_openai_codex_ws_lifecycle.py tests/test_proxy_loop_exception_health.py and uv run ruff format --check headroom/proxy/server.py tests/test_proxy_healthchecks.py tests/test_openai_codex_ws_lifecycle.py tests/test_proxy_loop_exception_health.py)
• Type checking passes (uv run mypy headroom)
• New tests added for new functionality when applicable
• Manual testing performed

Test Output

uv run pytest tests/test_proxy_healthchecks.py tests/test_openai_codex_ws_lifecycle.py tests/test_proxy_loop_exception_health.py -q

============================= test session starts =============================
platform win32 -- Python 3.12.13, pytest-9.0.3, pluggy-1.6.0
rootdir: D:\Repos\headroom-pr-1720-responses-ws-livez-wedge
configfile: pyproject.toml
plugins: anyio-4.12.1, langsmith-0.9.3, asyncio-1.3.0, cov-7.0.0
asyncio: mode=Mode.AUTO, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 33 items

tests\test_proxy_healthchecks.py ............                            [ 36%]
tests\test_openai_codex_ws_lifecycle.py ...................              [ 93%]
tests\test_proxy_loop_exception_health.py ..                             [100%]

============================== warnings summary ===============================
.venv\Lib\site-packages\fastapi\testclient.py:1
  D:\Repos\headroom-pr-1720-responses-ws-livez-wedge\.venv\Lib\site-packages\fastapi\testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
    from starlette.testclient import TestClient as TestClient  # noqa

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================== 33 passed, 1 warning in 9.78s ========================

uv run ruff check headroom/proxy/server.py tests/test_proxy_healthchecks.py tests/test_openai_codex_ws_lifecycle.py tests/test_proxy_loop_exception_health.py

All checks passed!

uv run ruff format --check headroom/proxy/server.py tests/test_proxy_healthchecks.py tests/test_openai_codex_ws_lifecycle.py tests/test_proxy_loop_exception_health.py

4 files already formatted

Real Behavior Proof

• Environment: Python proxy runtime with FastAPI TestClient, no external OpenAI credentials required.
• Exact command / steps: invoke the installed loop exception handler with an asyncio context matching Connection.connection_lost plus AttributeError("'ClientConnection' object has no attribute 'recv_messages'"), then request /livez.
• Observed result: the known websockets callback failure is recorded without delegating to the noisy default handler, /livez reports degraded loop callback health (HTTP 503, "status": "unhealthy", "alive": false), and unrelated callback exceptions still reach the delegated handler.
• Not tested: the nondeterministic upstream CPython or websockets timing edge against a live network connection; the focused regression pins the callback shape reported in [BUG] Proxy silently wedges (no crash, /livez hangs) after websockets connection_lost AttributeError #1720.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

Additional Notes

CHANGELOG.md is release-managed from conventional commits, so this PR does not edit it manually. The scope stays inside the proxy runtime and Codex WebSocket dispatch path; it does not change compression, CCR, provider-neutral pipeline behavior, or generic transform modules.

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.