add mongodb_store (#13801)

# Add MongoDB storage
  - **Description:** 
  Add MongoDB Storage as an option for large doc store. 

Example usage: 
```Python
# Instantiate the MongodbStore with a MongoDB connection
from langchain.storage import MongodbStore

mongo_conn_str = "mongodb://localhost:27017/"
mongodb_store = MongodbStore(mongo_conn_str, db_name="test-db",
                                collection_name="test-collection")

# Set values for keys
doc1 = Document(page_content='test1')
doc2 = Document(page_content='test2')
mongodb_store.mset([("key1", doc1), ("key2", doc2)])

# Get values for keys
values = mongodb_store.mget(["key1", "key2"])
# [doc1, doc2]

# Iterate over keys
for key in mongodb_store.yield_keys():
    print(key)

# Delete keys
mongodb_store.mdelete(["key1", "key2"])
 ```

  - **Dependencies:**
  Use `mongomock` for integration test.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>
Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/community/tests/integration_tests/storage/test_mongodb.py

@@ -0,0 +1,73 @@
+from typing import Generator
+
+import pytest
+from langchain_core.documents import Document
+
+from langchain_community.storage.mongodb import MongoDBStore
+
+pytest.importorskip("pymongo")
+
+
+@pytest.fixture
+def mongo_store() -> Generator:
+    import mongomock
+
+    # mongomock creates a mock MongoDB instance for testing purposes
+    with mongomock.patch(servers=(("localhost", 27017),)):
+        yield MongoDBStore("mongodb://localhost:27017/", "test_db", "test_collection")
+
+
+def test_mset_and_mget(mongo_store: MongoDBStore) -> None:
+    doc1 = Document(page_content="doc1")
+    doc2 = Document(page_content="doc2")
+
+    # Set documents in the store
+    mongo_store.mset([("key1", doc1), ("key2", doc2)])
+
+    # Get documents from the store
+    retrieved_docs = mongo_store.mget(["key1", "key2"])
+
+    assert retrieved_docs[0] and retrieved_docs[0].page_content == "doc1"
+    assert retrieved_docs[1] and retrieved_docs[1].page_content == "doc2"
+
+
+def test_yield_keys(mongo_store: MongoDBStore) -> None:
+    mongo_store.mset(
+        [
+            ("key1", Document(page_content="doc1")),
+            ("key2", Document(page_content="doc2")),
+            ("another_key", Document(page_content="other")),
+        ]
+    )
+
+    # Test without prefix
+    keys = list(mongo_store.yield_keys())
+    assert set(keys) == {"key1", "key2", "another_key"}
+
+    # Test with prefix
+    keys_with_prefix = list(mongo_store.yield_keys(prefix="key"))
+    assert set(keys_with_prefix) == {"key1", "key2"}
+
+
+def test_mdelete(mongo_store: MongoDBStore) -> None:
+    mongo_store.mset(
+        [
+            ("key1", Document(page_content="doc1")),
+            ("key2", Document(page_content="doc2")),
+        ]
+    )
+    # Delete single document
+    mongo_store.mdelete(["key1"])
+    remaining_docs = list(mongo_store.yield_keys())
+    assert "key1" not in remaining_docs
+    assert "key2" in remaining_docs
+
+    # Delete multiple documents
+    mongo_store.mdelete(["key2"])
+    remaining_docs = list(mongo_store.yield_keys())
+    assert len(remaining_docs) == 0
+
+
+def test_init_errors() -> None:
+    with pytest.raises(ValueError):
+        MongoDBStore("", "", "")