Add Responses API support

[toreleon]

Summary

• add /responses and /v1/responses proxy routes
• forward Responses API requests to the GitHub Copilot upstream responses endpoint
• add verbose logging and token-count reporting for Responses API requests

Validation

• bun run build
• manually verified gpt-5.3-codex and gpt-5.4-mini on /v1/responses through the local proxy