Add a new `/v1/responses` endpoint implementing the OpenResponses API
standard for agentic workflows. This provides:
- Item-based input (messages, function_call_output, reasoning)
- Semantic streaming events (response.created, response.output_text.delta,
response.completed, etc.)
- Full SSE event support with both event: and data: lines
- Configuration via gateway.http.endpoints.responses.enabled
The endpoint is disabled by default and can be enabled independently
from the existing Chat Completions endpoint.
Phase 1 implementation supports:
- String or ItemParam[] input
- system/developer/user/assistant message roles
- function_call_output items
- instructions parameter
- Agent routing via headers or model parameter
- Session key management
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
