fix(tests): add missing mocks for vision fallback and codex gateway tests

[crazywriter1]

Summary

Fixes 2 pre-existing test failures that affect main and all open PRs (e.g. #4245, #4246).

These tests pass locally in some environments but consistently fail in CI because they depend on runtime config resolution that behaves differently without a real config.yaml or hermes_cli setup.

Failing tests

1. test_vision_auto_falls_back_to_custom_endpoint

• File: tests/agent/test_auxiliary_client.py
• Error: assert None is not None
• Root cause: The test sets OPENAI_BASE_URL and OPENAI_API_KEY env vars expecting the vision auto-fallback to pick up the custom endpoint. However, _try_custom_endpoint() delegates to _resolve_custom_runtime() which imports hermes_cli.runtime_provider.resolve_runtime_provider — in CI this resolution returns (None, None) because there is no config file, so the custom endpoint is never constructed.
• Fix: Mock _resolve_custom_runtime to return the test endpoint values directly, matching what the test already sets via env vars.

2. test_gateway_run_agent_codex_path_handles_internal_401_refresh

• File: tests/test_codex_execution_paths.py
• Error: assert '⚠️ Codex Responses request 'model' must be a non-empty string.' == 'Recovered via refresh'
• Root cause: The test sets HERMES_MODEL env var but _run_agent() resolves the model via _resolve_gateway_model() which reads from config.yaml, not env vars. Without a config file, it returns an empty string — the codex API then rejects the request with a validation error before the 401-refresh logic is ever reached.
• Fix: Mock _resolve_gateway_model to return the test model string directly.

Changes

• tests/agent/test_auxiliary_client.py: Added _resolve_custom_runtime mock (+2 lines)
• tests/test_codex_execution_paths.py: Added _resolve_gateway_model mock (+1 line)

Test plan

• Both previously failing tests now pass locally
• No other tests affected (3 insertions only, no behavioral changes)