fix(google_ai): close aiohttp session to prevent ResourceWarning

Author: raymondWai

integrations/google_genai/src/haystack_integrations/components/generators/google_genai/chat/chat_generator.py

@@ -1013,3 +1013,5 @@ async def run_async(
 
             error_msg = f"Error in async Google Gen AI chat generation: {e}"
             raise RuntimeError(error_msg) from e
+        finally:
+            await self._client.aio.aclose()