fix: fix file open with encoding in chat_history.py (#31884)

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Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/core/langchain_core/chat_history.py

@@ -65,28 +65,32 @@ class BaseChatMessageHistory(ABC):
         .. code-block:: python
 
             class FileChatMessageHistory(BaseChatMessageHistory):
-                storage_path:  str
+                storage_path: str
                 session_id: str
 
-               @property
-               def messages(self):
-                   with open(os.path.join(storage_path, session_id), 'r:utf-8') as f:
-                       messages = json.loads(f.read())
+                @property
+                def messages(self):
+                    with open(
+                        os.path.join(storage_path, session_id),
+                        "r",
+                        encoding="utf-8",
+                    ) as f:
+                        messages = json.loads(f.read())
                     return messages_from_dict(messages)
 
-               def add_messages(self, messages: Sequence[BaseMessage]) -> None:
-                   all_messages = list(self.messages) # Existing messages
-                   all_messages.extend(messages) # Add new messages
+                def add_messages(self, messages: Sequence[BaseMessage]) -> None:
+                    all_messages = list(self.messages)  # Existing messages
+                    all_messages.extend(messages)  # Add new messages
 
-                   serialized = [message_to_dict(message) for message in all_messages]
-                   # Can be further optimized by only writing new messages
-                   # using append mode.
-                   with open(os.path.join(storage_path, session_id), 'w') as f:
-                       json.dump(f, messages)
+                    serialized = [message_to_dict(message) for message in all_messages]
+                    # Can be further optimized by only writing new messages
+                    # using append mode.
+                    with open(os.path.join(storage_path, session_id), "w") as f:
+                        json.dump(messages, f)
 
-               def clear(self):
-                   with open(os.path.join(storage_path, session_id), 'w') as f:
-                       f.write("[]")
+                def clear(self):
+                    with open(os.path.join(storage_path, session_id), "w") as f:
+                        f.write("[]")
     """
 
     messages: list[BaseMessage]