fix: Avoid pydantic.ValidationError when the model stream returns empty final chunk

Bug: When a model emits a stream of tokens, it sometimes emits a final chunk of whitespace or no content. The agent was trying to parse that content into JSON, causing a validation error.

Fix: If a model is expected to return JSON and the last streamed token is empty/whitespace, the agent will no longer try to parse it, and exit gracefully.

New unit tests confirm the scenario and the fix.

PiperOrigin-RevId: 777609415

src/google/adk/agents/llm_agent.py

@@ -451,6 +451,11 @@ class LlmAgent(BaseAgent):
           [part.text if part.text else '' for part in event.content.parts]
       )
       if self.output_schema:
+        # If the result from the final chunk is just whitespace or empty,
+        # it means this is an empty final chunk of a stream.
+        # Do not attempt to parse it as JSON.
+        if not result.strip():
+          return
         result = self.output_schema.model_validate_json(result).model_dump(
             exclude_none=True
         )