feat: add OpenAI Responses API endpoint support

[Alex1Yuan]

Summary

• Add /v1/responses and /responses endpoints that forward requests directly to GitHub Copilot's responses API
• Enables support for responses-only models (gpt-5.4, gpt-5.3-codex, etc.)
• Follows existing project patterns: rate limiting, manual approval, vision detection, X-Initiator header

Changes

• src/services/copilot/create-responses.ts — Copilot responses API client
• src/routes/responses/route.ts — Route registration with error handling
• src/routes/responses/handler.ts — Request handler (streaming + non-streaming)
• src/server.ts — Register new endpoints

Test plan

• TypeScript type check passes (tsc --noEmit)
• ESLint passes (pre-commit hook)
• Tested gpt-5.4 non-streaming — returns correct response
• Tested gpt-5.4 streaming — full SSE event chain works
• Tested gpt-5.3-codex non-streaming — returns correct response
• Verified gpt-4o correctly returns unsupported_api_for_model error
• Existing endpoints unaffected (backward compatible)

[caozhiyuan]

@Alex1Yuan Duplicated with #170. You can use https://github.com/caozhiyuan/copilot-api (includes many advanced features). This repository hasn't been maintained for half a year.

[Alex1Yuan]

@Alex1Yuan Duplicated with #170. You can use https://github.com/caozhiyuan/copilot-api (includes many advanced features). This repository hasn't been maintained for half a year.

Get it, thank you.