fix(aux): keep xai oauth auxiliary on chat completions

[LeonSGP43]

Fixes #34171.

Summary

• keep xAI OAuth auxiliary clients on native chat completions instead of wrapping them in the Responses adapter
• preserve the existing explicit-model and unauthenticated fallthrough behavior
• update the xAI OAuth routing regression test to pin the chat-completions contract

Testing

• uv run --frozen pytest -q -o addopts='' tests/hermes_cli/test_auth_xai_oauth_provider.py -k "auxiliary_client_routes_xai_oauth_through_chat_completions_api or auxiliary_client_xai_oauth_returns_none_when_unauthenticated or auxiliary_client_xai_oauth_requires_explicit_model"
• uv run --frozen ruff check agent/auxiliary_client.py tests/hermes_cli/test_auth_xai_oauth_provider.py
• git diff --check

[teknium1]

Thanks for the focused auxiliary-routing investigation. Current main now has an explicit, conflicting xAI OAuth transport contract.

Problems

• agent/auxiliary_client.py:2429-2452 intentionally wraps xAI OAuth auxiliary clients in CodexAuxiliaryClient; tests/hermes_cli/test_auth_xai_oauth_provider.py:1840-1875 asserts that contract.
• The main resolver independently selects api_mode="codex_responses" for xAI OAuth in hermes_cli/runtime_provider.py:421-423. Returning a plain client only for auxiliaries would diverge from the active provider route and bypass xAI Responses handling in agent/auxiliary_client.py:950-1008.

Suggested changes

• Please reproduce the 403 using the current device-code OAuth flow before changing this contract.
• If it persists, update the shared routing/adapter behavior coherently and retain equivalent schema-sanitization and cache behavior with an end-to-end regression.

Automated hermes-sweeper review.