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

