fix: Fix the return type of generate_content_stream.

PiperOrigin-RevId: 805343984

Author: google-genai-bot

google/genai/models.py

@@ -8342,7 +8342,7 @@ async def generate_content_stream(
       model: str,
       contents: Union[types.ContentListUnion, types.ContentListUnionDict],
       config: Optional[types.GenerateContentConfigOrDict] = None,
-  ) -> AsyncIterator[types.GenerateContentResponse]:
+  ) -> Awaitable[AsyncIterator[types.GenerateContentResponse]]:
     """Makes an API request to generate content using a model and yields the model's response in chunks.
 
     For the `model` parameter, supported formats for Vertex AI API include: