[FEATURE]: Support session-scoped streaming responses for message submission #17020

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Describe the enhancement you want to request

Currently, there is no efficient way to receive real-time updates for a specific session without significant overhead. The existing two methods are:

POST /session/:id/message: Synchronous and blocking. This is unsuitable for production as it risks timeouts and consumes server-side threads for long-running tasks.

POST /session/:id/prompt_async: Asynchronous (204), but results are broadcasted via the global /event endpoint. This forces the client to either filter a high-volume stream of all sessions' data or fallback to inefficient polling via GET /session/:id/message.

The Proposal:
I suggest introducing a Stream API (Server-Sent Events) that allows users to receive updates for a specific session directly.

Desired Behavior:

A new endpoint (e.g., POST /session/:id/message/stream or GET /session/:id/events) that returns text/event-stream.
The stream should only contain events relevant to the specific :id.
Alternatively, allow the POST request to return a stream directly as the response body.

Why this matters:

1. Performance: Reduces client-side CPU usage (no need to filter global noise).
2. Scalability: Eliminates the need for frequent polling.
3. UX: Enables low-latency, "typewriter" style responses for a better user experience.