langchain/memory/chat_message_histories/firestore.py/FirestoreChatMessageHistory/load_messages
class FirestoreChatMessageHistory:    def load_messages(self) -> None:
        """Retrieve the messages from Firestore"""
        if not self._document:
            raise ValueError("Document not initialized")
        doc = self._document.get()
        if doc.exists:
            data = doc.to_dict()
            if "messages" in data and len(data["messages"]) > 0:
                self.messages = messages_from_dict(data["messages"])