GitHub Copilot's /responses endpoint only serves OpenAI (gpt/codex)
models. gemini-3.1-pro-preview was failing on /chat/completions with a
"not supported" error, getting cached as a codex model, then escalated
to /responses where it 400s with "does not support Responses API".
Add GithubExecutor.supportsResponsesEndpoint() and gate both the cached
/responses route and the 400-fallback on it, so Gemini/Claude always
stay on /chat/completions and the real upstream error surfaces.
Adds tests/unit/github-responses-routing.test.js (5 tests).
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
