Fix Codex Responses crash when output is null

[TriMinhPham]

Summary

• detect the OpenAI SDK Responses stream parser crash caused by response.output = null
• recover from streamed Codex output items or text deltas when available
• normalize remaining output = null cases into the existing malformed-response retry/fallback path
• guard response.output_text access because the SDK property can raise the same TypeError when output is null

Tests

• venv/bin/python -m pytest tests/run_agent/test_run_agent_codex_responses.py
• venv/bin/python -m pytest tests/agent/transports/test_codex_transport.py
• venv/bin/python -m py_compile agent/codex_runtime.py agent/codex_responses_adapter.py agent/conversation_loop.py

[teknium1]

Closing as duplicate — the Codex null-output fix has been merged via #32963 (cherry-picked from @carltonawong's PR #32890). Thanks for the help during the outage. Closes #11179.