langchain/memory/chat_message_histories/zep.py/ZepChatMessageHistory/zep_summary
class ZepChatMessageHistory:    def zep_summary(self) -> Optional[str]:
        """Retrieve summary from Zep memory"""
        zep_memory: Optional[Memory] = self._get_memory()
        if not zep_memory or not zep_memory.summary:
            return None

        return zep_memory.summary.content