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/tests/test_brains.py

@@ -498,6 +498,25 @@ class WorkflowBrainTests(unittest.IsolatedAsyncioTestCase):
         start_brain._manager = FakeManager()
         await start_brain.on_connected()
         self.assertEqual(start_brain._manager.current_node, "start")
+        queued.clear()
+        await start_brain.on_client_ready()
+        ready_messages = [
+            frame.message
+            for frame in queued
+            if isinstance(frame, OutputTransportMessageUrgentFrame)
+        ]
+        self.assertIn(
+            {"type": "node-active", "nodeId": "start"},
+            ready_messages,
+        )
+        self.assertTrue(
+            any(
+                message.get("type") == "workflow-variables"
+                and message.get("reason") == "client_ready"
+                and message.get("nodeId") == "start"
+                for message in ready_messages
+            )
+        )
 
         agent_brain = WorkflowBrain(
             {
@@ -524,6 +543,16 @@ class WorkflowBrainTests(unittest.IsolatedAsyncioTestCase):
         agent_brain._runtime = runtime
         agent_brain._manager = FakeManager("agent")
         queued.clear()
+        await agent_brain.on_client_ready()
+        self.assertTrue(
+            any(
+                isinstance(frame, OutputTransportMessageUrgentFrame)
+                and frame.message
+                == {"type": "node-active", "nodeId": "agent"}
+                for frame in queued
+            )
+        )
+        queued.clear()
         handled = await agent_brain.on_user_turn_end("请继续回答")
         self.assertTrue(handled)
         self.assertEqual(agent_brain._manager.current_node, "agent")