Add a test for cosmos db memory (#3525)

Test for #3434 @eavanvalkenburg 
Initially, I was unaware and had submitted a pull request #3450 for the
same purpose, but I have now repurposed the one I used for that. And it
worked.

tests/integration_tests/memory/test_cosmos_db.py

@@ -0,0 +1,44 @@
+import json
+import os
+
+from langchain.memory import ConversationBufferMemory
+from langchain.memory.chat_message_histories import CosmosDBChatMessageHistory
+from langchain.schema import _message_to_dict
+
+# Replace these with your Azure Cosmos DB endpoint and key
+endpoint = os.environ["COSMOS_DB_ENDPOINT"]
+credential = os.environ["COSMOS_DB_KEY"]
+
+
+def test_memory_with_message_store() -> None:
+    """Test the memory with a message store."""
+    # setup Azure Cosmos DB as a message store
+    message_history = CosmosDBChatMessageHistory(
+        cosmos_endpoint=endpoint,
+        cosmos_database="chat_history",
+        cosmos_container="messages",
+        credential=credential,
+        session_id="my-test-session",
+        user_id="my-test-user",
+        ttl=10,
+    )
+    message_history.prepare_cosmos()
+    memory = ConversationBufferMemory(
+        memory_key="baz", chat_memory=message_history, return_messages=True
+    )
+
+    # add some messages
+    memory.chat_memory.add_ai_message("This is me, the AI")
+    memory.chat_memory.add_user_message("This is me, the human")
+
+    # get the message history from the memory store and turn it into a json
+    messages = memory.chat_memory.messages
+    messages_json = json.dumps([_message_to_dict(msg) for msg in messages])
+
+    assert "This is me, the AI" in messages_json
+    assert "This is me, the human" in messages_json
+
+    # remove the record from Azure Cosmos DB, so the next test run won't pick it up
+    memory.chat_memory.clear()
+
+    assert memory.chat_memory.messages == []