Add on_client_ready method to BaseBrain and WorkflowBrain

- Implement on_client_ready in BaseBrain to handle client-visible state after the app message channel is ready.
- Extend WorkflowBrain with on_client_ready to replay state that may have been emitted before WebRTC data was ready.
- Update pipeline to call on_client_ready when a client connects.
- Enhance tests to verify the correct behavior of on_client_ready in WorkflowBrain.

backend/services/pipecat/pipeline.py

@@ -1218,6 +1218,7 @@ async def run_pipeline(
         if greeting and not greeting_transcript_sent:
             greeting_transcript_sent = True
             await queue_transcript("assistant", greeting, time_now_iso8601())
+        await brain.on_client_ready()
 
     @transport.event_handler("on_client_connected")
     async def on_client_connected(_transport, _client):