Fix is_complete

Author: DouweM

pydantic_ai_slim/pydantic_ai/result.py

@@ -787,7 +787,7 @@ def is_complete(self) -> bool:
         [`stream_responses`][pydantic_ai.result.StreamedRunResultSync.stream_responses] or
         [`get_output`][pydantic_ai.result.StreamedRunResultSync.get_output] completes.
         """
-        return self._streamed_run_result.is_complete
+        return self._async_to_sync(lambda result: result.is_complete)
 
 
 @dataclass(repr=False)