Open a Responses WebSocket

Upgrades to a WebSocket for multi-turn Responses API generation. Send UTF-8 JSON `response.create` messages and receive `ResponseStreamEvent` messages. The `stream` field is accepted but WebSocket turns always emit events. The `background` field must not be sent. Ping frames receive pong frames. Authenticate with a Bearer API key or console user session. Clients that cannot set the `Authorization` header may send the legacy `modelsocket-api-key.<API_KEY>` authentication value in `Sec-WebSocket-Protocol`.