Fix serialization issue in Matching Engine Vector Store (#13266)

- **Description:** Fixed a serialization issue in the add_texts method
of the Matching Engine Vector Store caused by a typo, leading to an
attempt to serialize the json module itself.
  - **Issue:** #12154 
  - **Dependencies:** ./.
  - **Tag maintainer:**

libs/langchain/langchain/vectorstores/matching_engine.py

@@ -137,7 +137,7 @@ class MatchingEngine(VectorStore):
             json_: dict = {"id": id, "embedding": embedding}
             if metadatas is not None:
                 json_["metadata"] = metadatas[idx]
-            jsons.append(json)
+            jsons.append(json_)
             self._upload_to_gcs(text, f"documents/{id}")
 
         logger.debug(f"Uploaded {len(ids)} documents to GCS.")