fix: Exclude thought parts when merging agent output

PiperOrigin-RevId: 850500329

src/google/adk/tools/agent_tool.py

@@ -188,7 +188,9 @@ class AgentTool(BaseTool):
 
     if not last_content:
       return ''
-    merged_text = '\n'.join(p.text for p in last_content.parts if p.text)
+    merged_text = '\n'.join(
+        p.text for p in last_content.parts if p.text and not p.thought
+    )
     if isinstance(self.agent, LlmAgent) and self.agent.output_schema:
       tool_result = self.agent.output_schema.model_validate_json(
           merged_text