fix(ai): omit service tier for GitHub Copilot

[traditio]

Summary

GitHub Copilot rejects the OpenAI Responses service_tier field. Prime Agent currently supplies "default" when no tier is configured, causing requests to fail with HTTP 400.

• Omit service_tier for GitHub Copilot Responses requests.
• Preserve configured service tiers for supported providers.
• Cover both request paths with regression tests.

Testing

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/openai-responses-copilot-provider.test.ts (25 passed)
• npm run check

Note

Omit service_tier from GitHub Copilot request payloads in openai-responses provider

GitHub Copilot does not support the service_tier field, so including it in requests causes errors. The buildParams function in openai-responses.ts now skips setting service_tier when model.provider is 'github-copilot', while other providers (e.g. OpenAI) continue to receive it when serviceTier is specified.

^{Macroscope summarized 704cb12.}