Observe Async Chat Stream Events

Observe an asynchronous chat stream via Server-Sent Events. This endpoint provides a Server-Sent Events stream that delivers chat completion events in real-time. The events follow the same format as synchronous chat streaming responses. Stream Lifecycle: 1. Stream initiated via POST /v1/messages/async 2. Events begin flowing when processing starts 3. Stream automatically closes when message completes 4. Stream can be cancelled via POST /v1/messages/async/{message_id}/cancel Notes: * Stream remains active until message completion or cancellation * Events are delivered in chronological order * Connection will automatically close when stream ends * Use appropriate SSE client libraries for robust event handling