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/brains/base.py

@@ -86,6 +86,9 @@ class BaseBrain:
     async def on_connected(self) -> None:
         """Handle a connected client after the common greeting is queued."""
 
+    async def on_client_ready(self) -> None:
+        """Replay client-visible state after its app message channel is ready."""
+
     def record_user_message(self, content: str) -> None:
         """Observe a committed user message for brain-owned routing state."""
 
@@ -126,6 +129,8 @@ class Brain(Protocol):
 
     async def on_connected(self) -> None: ...
 
+    async def on_client_ready(self) -> None: ...
+
     def record_user_message(self, content: str) -> None: ...
 
     async def on_user_turn_end(self, content: str) -> bool: ...