fix(ai): route Copilot Grok 4.5 to responses

[jay-tau]

Summary

• Route GitHub Copilot grok-4.5 through the OpenAI Responses API.
• Regenerate the Copilot model catalog and add regression coverage.

Testing

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/github-copilot-models.test.ts
• npm run check

Fixes #875

Note

Route GitHub Copilot grok-4.5 to the Responses API instead of Chat Completions

The grok-4.5 model under the github-copilot provider does not support the Chat Completions endpoint, so it is now assigned openai-responses in generate-models.ts alongside the existing gpt-5 and oswe models. The generated catalog and a new test in github-copilot-models.test.ts confirm the correct routing.

^{Macroscope summarized 76b84a2.}