fix(vectorstores.py): fetch embedding for the near_vector method

the near_vector method accepts a parameter called "near_vector", for which the method will perform vector search to get the nearest vector(s). We should get the vector using the embedding object, just as we did in the `hybrid` search method, 4 lines above it.

langchain_weaviate/vectorstores.py

@@ -255,7 +255,12 @@ def _perform_search(
                         query=query, vector=embedding, limit=k, **kwargs
                     )
                 elif search_method == "near_vector":
-                    result = collection.query.near_vector(limit=k, **kwargs)
+                    embedding = self._embedding.embed_query(query)
+                    result = collection.query.near_vector(
+                        near_vector = embedding, 
+                        limit=k, 
+                        **kwargs
+                    )
                 else:
                     raise ValueError(f"Invalid search method: {search_method}")
             except weaviate.exceptions.WeaviateQueryException as e: