fix: stabilize codex oauth runtime in packaged installs

[typelicious]

Summary

• stop dropping OAuth-backed Codex providers at startup when no static api_key is present
• resolve faigate-auth reliably in Brew/libexec installs and preserve explicit empty Codex chat_path values
• route Codex stream=true requests through the same responses adapter and cover the packaged runtime path with tests

Validation

• rtk ruff check --no-cache faigate/main.py faigate/oauth/backend.py faigate/providers.py tests/test_client_profiles.py tests/test_providers.py
• python3 -m pytest -q tests/test_providers.py tests/test_client_profiles.py
• local smoke test with live config on a temporary Gate instance for stream=false and stream=true via faigate/openai-codex-5.4-medium