Support chaining for tool callbacks

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

PiperOrigin-RevId: 756526507

contributing/samples/hello_world/agent.py

@@ -117,6 +117,31 @@ def before_agent_cb3(callback_context):
   print('@before_agent_cb3')
 
 
+def before_tool_cb1(tool, args, tool_context):
+  print('@before_tool_cb1')
+
+
+def before_tool_cb2(tool, args, tool_context):
+  print('@before_tool_cb2')
+
+
+def before_tool_cb3(tool, args, tool_context):
+  print('@before_tool_cb3')
+
+
+def after_tool_cb1(tool, args, tool_context, tool_response):
+  print('@after_tool_cb1')
+
+
+def after_tool_cb2(tool, args, tool_context, tool_response):
+  print('@after_tool_cb2')
+  return {'test': 'after_tool_cb2', 'response': tool_response}
+
+
+def after_tool_cb3(tool, args, tool_context, tool_response):
+  print('@after_tool_cb3')
+
+
 root_agent = Agent(
     model='gemini-2.0-flash-exp',
     name='data_processing_agent',
@@ -166,4 +191,6 @@ root_agent = Agent(
     after_agent_callback=[after_agent_cb1, after_agent_cb2, after_agent_cb3],
     before_model_callback=before_model_callback,
     after_model_callback=after_model_callback,
+    before_tool_callback=[before_tool_cb1, before_tool_cb2, before_tool_cb3],
+    after_tool_callback=[after_tool_cb1, after_tool_cb2, after_tool_cb3],
 )