Add SSE streaming support to Python SDK

[sunilp]

Summary

The Python SDK's LdpClient.submit_task() currently blocks until the task completes. For long-running tasks, clients need to stream progress updates via Server-Sent Events (SSE).

What to do

1. Add stream_task() method to LdpClient that returns an async iterator of TaskEvent
2. Add SSE endpoint to LdpDelegate at /ldp/stream/{task_id}
3. Yield TaskEvent.Progress, TaskEvent.Completed, or TaskEvent.Failed events
4. Add on_progress callback support in LdpDelegate.handle_task()

Example API

# Client side
async for event in client.stream_task(url, skill="reasoning", input_data={...}):
    if event.type == "progress":
        print(f"Progress: {event.progress}%")
    elif event.type == "completed":
        print(f"Result: {event.output}")

# Delegate side
class MyDelegate(LdpDelegate):
    async def handle_task(self, skill, input_data, task_id):
        await self.send_progress(task_id, 0.25, "Analyzing...")
        await self.send_progress(task_id, 0.75, "Synthesizing...")
        return {"answer": "done"}, 0.92

Why this matters

Streaming is table-stakes for production agent systems. Without it, long tasks appear frozen from the client's perspective. A2A supports streaming via SSE — LDP should too.

References

• Current polling implementation: src/adapter.rs (Rust, lines 195-254)
• SSE spec: https://html.spec.whatwg.org/multipage/server-sent-events.html
• httpx SSE: https://www.python-httpx.org/advanced/sse/

Acceptance criteria

• client.stream_task() returns an async iterator
• Delegate can send progress updates during task execution
• Works with the existing message protocol (TASK_UPDATE messages)
• Integration test covering the streaming flow