langchain-mongodb: [test-fix] add explicit index_name setting on test vector creation (#19245)

- **Description:** Tests fail to do value lookup because it does not
specify the index name
  - **Issue:** the issue # Failing integration test
 

- [x] **Add tests and docs**: Tests now pass


- [x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

libs/partners/mongodb/langchain_mongodb/cache.py

@@ -215,6 +215,7 @@ class MongoDBAtlasSemanticCache(BaseCache, MongoDBAtlasVectorSearch):
         embedding: Embeddings,
         collection_name: str = "default",
         database_name: str = "default",
+        index_name: str = "default",
         wait_until_ready: bool = False,
         **kwargs: Dict[str, Any],
     ):
@@ -229,13 +230,20 @@ class MongoDBAtlasSemanticCache(BaseCache, MongoDBAtlasVectorSearch):
                 Defaults to "default".
             database_name (str): MongoDB Database where to store texts.
                 Defaults to "default".
+            index_name: Name of the Atlas Search index.
+                defaults to 'default'
             wait_until_ready (bool): Block until MongoDB Atlas finishes indexing
                 the stored text. Hard timeout of 10 seconds. Defaults to False.
         """
         client = _generate_mongo_client(connection_string)
         self.collection = client[database_name][collection_name]
         self._wait_until_ready = wait_until_ready
-        super().__init__(self.collection, embedding, **kwargs)  # type: ignore
+        super().__init__(
+            collection=self.collection,
+            embedding=embedding,
+            index_name=index_name,
+            **kwargs,  # type: ignore
+        )
 
     def lookup(self, prompt: str, llm_string: str) -> Optional[RETURN_VAL_TYPE]:
         """Look up based on prompt and llm_string."""

libs/partners/mongodb/tests/integration_tests/test_cache.py

@@ -29,6 +29,7 @@ def llm_cache(cls: Any) -> BaseCache:
             connection_string=CONN_STRING,
             collection_name=COLLECTION,
             database_name=DATABASE,
+            index_name=INDEX_NAME,
             wait_until_ready=True,
         )
     )