fix linting

libs/genai/langchain_google_genai/chat_models.py

@@ -341,15 +341,17 @@ def _parse_chat_history(
     return messages
 
 
-def _parts_to_content(parts: List[genai.types.PartType]) -> Union[List[dict], str]:
+def _parts_to_content(
+    parts: List[genai.types.PartType],
+) -> Union[str, List[Union[str, dict]]]:
     """Converts a list of Gemini API Part objects into a list of LangChain messages."""
     if len(parts) == 1 and parts[0].text is not None and not parts[0].inline_data:
         # Simple text response. The typical response
         return parts[0].text
     elif not parts:
         logger.warning("Gemini produced an empty response.")
         return ""
-    messages = []
+    messages: List[Union[str, dict]] = []
     for part in parts:
         if part.text is not None:
             messages.append(