Responses Endpoint

OpenAI Responses API surface.

Workers fall into two camps:

1. Native Responses handler — returns real Responses shape + response.* SSE events. Forwarded straight through.
2. Chat-only worker — body is translated to chat-completions shape, hits /v1/chat/completions, then the chat-completion JSON is converted back to Responses shape on the way out. Streaming requests pass the chat.completion.chunk SSE through verbatim (a separate refactor will rewrite chunks into response.* events).