Step 3 of the Responses rollout: plain proxy route on the gateway,
no translation. Neuron speaks the Responses API natively after Step
2 (commit 957f704), so the gateway just needs the same routing
shape it uses for /v1/chat/completions — extract `model`, resolve
via router::resolve, forward verbatim.
- New `POST /v1/responses` handler in handlers.rs::responses.
- Mock neuron under tests/common/mod.rs gains a `/v1/responses`
endpoint that mirrors the ResponsesResponse shape neuron emits.
- New integration test file `tests/responses.rs` exercises:
- Happy path (200, body round-trips, ResponsesUsage shape).
- Unknown model → 404 (matches chat-completions error shape).
- Missing `model` field → 400 (same extract_model helper).
Streaming proxy works through the same path as chat completions —
the upstream Content-Type (`text/event-stream` for stream:true,
`application/json` otherwise) propagates through proxy_with_metrics
unchanged. Live-stream integration tests against a streaming mock
deferred until we exercise the path against a real neuron, since
the chat-completions streaming test already covers the proxy's
SSE forwarding mechanics.
Three new tests; clippy + fmt clean across the workspace.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
