Fix an issue with chat request validation for LangChain flavor (mlflo…

…w#11478)

Signed-off-by: Ben Wilson <[email protected]>

mlflow/langchain/api_request_parallel_processor.py

@@ -285,13 +285,14 @@ def _transform_request_json_for_chat_if_necessary(request_json, lc_model):
             # is handling the "messages" field by itself
             return request_json, False
 
-        def json_dict_might_be_chat_request(json: Dict):
+        def json_dict_might_be_chat_request(json_message: Dict):
             return (
-                "messages" in request_json
+                isinstance(json_message, dict)
+                and "messages" in json_message
                 and
                 # Additional keys can't be specified when calling LangChain invoke() / batch()
                 # with chat messages
-                len(request_json) == 1
+                len(json_message) == 1
             )
 
         if isinstance(request_json, dict) and json_dict_might_be_chat_request(request_json):