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/workflow_brain.py

@@ -123,6 +123,19 @@ class WorkflowBrain(BaseBrain):
         await self._manager.initialize(node_config)
         logger.info(f"工作流模式启用: 当前节点={self._manager.current_node}")
 
+    async def on_client_ready(self) -> None:
+        """Replay state that may have been emitted before WebRTC data was ready."""
+        current_node = (
+            str(self._manager.current_node)
+            if self._manager and self._manager.current_node
+            else self._engine.start_id
+        )
+        await self._emit_node_active(current_node)
+        await self._emit_variables(
+            reason="client_ready",
+            node_id=current_node,
+        )
+
     def record_user_message(self, content: str) -> None:
         if content and not self._ended:
             self._store.record("user", content)