fix: Filter out thought parts when saving agent output to state

PiperOrigin-RevId: 809270320

src/google/adk/agents/llm_agent.py

@@ -515,7 +515,9 @@ class LlmAgent(BaseAgent):
     ):
 
       result = ''.join(
-          [part.text if part.text else '' for part in event.content.parts]
+          part.text
+          for part in event.content.parts
+          if part.text and not part.thought
       )
       if self.output_schema:
         # If the result from the final chunk is just whitespace or empty,