Support chaining for agent callbacks

(before/after) agent callbacks are invoked throughout the provided chain until one callback does not return None. Callbacks can be async and sync.

PiperOrigin-RevId: 756359693

contributing/samples/hello_world/agent.py

@@ -86,6 +86,37 @@ async def after_model_callback(callback_context, llm_response):
   return None
 
 
+def after_agent_cb1(callback_context):
+  print('@after_agent_cb1')
+
+
+def after_agent_cb2(callback_context):
+  print('@after_agent_cb2')
+  return types.Content(
+      parts=[
+          types.Part(
+              text='(stopped) after_agent_cb2',
+          ),
+      ],
+  )
+
+
+def after_agent_cb3(callback_context):
+  print('@after_agent_cb3')
+
+
+def before_agent_cb1(callback_context):
+  print('@before_agent_cb1')
+
+
+def before_agent_cb2(callback_context):
+  print('@before_agent_cb2')
+
+
+def before_agent_cb3(callback_context):
+  print('@before_agent_cb3')
+
+
 root_agent = Agent(
     model='gemini-2.0-flash-exp',
     name='data_processing_agent',
@@ -127,8 +158,12 @@ root_agent = Agent(
             ),
         ]
     ),
-    before_agent_callback=before_agent_callback,
-    after_agent_callback=after_agent_callback,
+    before_agent_callback=[
+        before_agent_cb1,
+        before_agent_cb2,
+        before_agent_cb3,
+    ],
+    after_agent_callback=[after_agent_cb1, after_agent_cb2, after_agent_cb3],
     before_model_callback=before_model_callback,
     after_model_callback=after_model_callback,
 )