fix: AgentTool returns last content, instead of the content in the last event

This fixes the issue when inner agent has after_agent_callback updating state only, AgentTool will always return empty. PiperOrigin-RevId: 799814933

Author: google-genai-bot

src/google/adk/tools/agent_tool.py

@@ -139,7 +139,7 @@ async def run_async(
  state=tool_context.state.to_dict(),
  )
 
- last_event = None
+ last_content = None
  async with Aclosing(
  runner.run_async(
  user_id=session.user_id, session_id=session.id, new_message=content
@@ -149,11 +149,12 @@ async def run_async(
  # Forward state delta to parent session.
  if event.actions.state_delta:
  tool_context.state.update(event.actions.state_delta)
- last_event = event
+ if event.content:
+ last_content = event.content
 
- if not last_event or not last_event.content or not last_event.content.parts:
+ if not last_content:
  return ''
- merged_text = '\n'.join(p.text for p in last_event.content.parts if p.text)
+ merged_text = '\n'.join(p.text for p in last_content.parts if p.text)
  if isinstance(self.agent, LlmAgent) and self.agent.output_schema:
  tool_result = self.agent.output_schema.model_validate_json(
  merged_text