Use symmetric spawn-then-run() pattern in multi-task examples

Switch every example to ``await runner.spawn(task)`` followed by
``await runner.run()`` (no task argument), and ``await runner.cancel()``
on client-disconnected instead of ``await task.cancel()``. This makes
the main pipeline task look the same as the worker / proxy tasks
spawned alongside it, and lets ``runner.cancel()`` drive a uniform
shutdown across every root task on the bus.

examples/multi-task/remote-proxy-assistant/assistant.py

@@ -104,8 +104,10 @@ async def websocket_endpoint(websocket: WebSocket):
     assistant = AcmeAssistant()
 
     await runner.spawn(proxy)
+    await runner.spawn(assistant)
+
     logger.info("Assistant server ready, waiting for activation")
-    await runner.run(assistant)
+    await runner.run()
     logger.info("Assistant server session ended")