Add role to the event returned in agent callback sample

Events without a role are filtered out in the next turns.

PiperOrigin-RevId: 758393355

contributing/samples/callbacks/agent.py

@@ -92,7 +92,9 @@ def after_agent_cb1(callback_context):
 
 def after_agent_cb2(callback_context):
   print('@after_agent_cb2')
-  return types.Content(
+  # ModelContent (or Content with role set to 'model') must be returned.
+  # Otherwise, the event will be excluded from the context in the next turn.
+  return types.ModelContent(
       parts=[
           types.Part(
               text='(stopped) after_agent_cb2',