Add WebSocket transport for OpenAI Responses API streaming

[dgageot]

Introduce an optional WebSocket transport as an alternative to SSE for the OpenAI Responses API. Users can enable it via provider_opts:

models:
  gpt-ws:
    provider: openai
    model: gpt-4.1
    provider_opts:
      transport: websocket  # Use WebSocket instead of SSE

Key changes:

• Add responseEventStream interface to abstract SSE and WebSocket transports
• Refactor ResponseStreamAdapter to accept any responseEventStream
• Implement wsStream (WebSocket transport) and wsPool (connection pool with 55-min TTL, auto-reconnect, and lastResponseID tracking)
• Integrate WebSocket path in CreateResponseStream with automatic SSE fallback on connection failure
• No new dependencies (reuses existing gorilla/websocket)

The existing ResponseStreamAdapter.Recv() logic is fully reused since WebSocket events use the same JSON schema as SSE events.

Assisted-By: docker-agent

[dgageot]

/review

[docker-agent]

Assessment: 🟢 APPROVE

This PR introduces WebSocket transport for the OpenAI Responses API with solid implementation and comprehensive tests. The code is well-structured with proper connection pooling, error handling, and automatic SSE fallback.

One minor issue was found: the lastResponseID field is tracked but never used for server-side context caching as intended by its documentation.

[docker-agent]

ISSUE: Incomplete Feature Implementation

The lastResponseID field is tracked but never used for server-side context caching.

Details:
The field is declared with documentation stating "It can be passed as previous_response_id in subsequent requests to enable server-side context caching" (line 42-43), and it's correctly updated when terminal events are received (line 184 in pooledStream.Next()). However, there's no code that reads lastResponseID and passes it as previous_response_id in the request params.

Impact:
The server-side context caching feature mentioned in the documentation is non-functional. Subsequent requests on the same pooled connection won't benefit from cached context.

Recommendation:
Either:

1. Implement the feature by reading p.lastResponseID in wsPool.Stream() and injecting it into the params before sending the request
2. Remove the field and its documentation if context caching is not needed yet