Add missing async declaration

app.py

@@ -50,7 +50,7 @@ class BasicAgent:
         else:
             return TextBlock(text=item)
 
-    def __call__(self, messages) -> str:
+    async def __call__(self, messages) -> str:
         print(f"Agent received question (first 50 chars): {question[:50]}...")
         blocks = list(map(create_block, messages))
         request = ChatMessage(role="user", blocks=blocks)