Refa: add result to callback for agent tool use. (#9137)

### What problem does this PR solve?


### Type of change

- [x] Refactoring

agent/component/message.py

@@ -143,4 +143,4 @@ class Message(ComponentBase):
         self.set_output("content", content)
 
     def thoughts(self) -> str:
-        return "Thinking ..."
+        return ""