langchain/memory/buffer.py/ConversationStringBufferMemory/validate_chains
class ConversationStringBufferMemory:    def validate_chains(cls, values: Dict) -> Dict:
        """Validate that return messages is not True."""
        if values.get("return_messages", False):
            raise ValueError(
                "return_messages must be False for ConversationStringBufferMemory"
            )
        return values