Fix/abstract add message (#8856)

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libs/langchain/langchain/schema/memory.py

@@ -107,14 +107,14 @@ class BaseChatMessageHistory(ABC):
         """
         self.add_message(AIMessage(content=message))
 
-    # TODO: Make this an abstractmethod.
+    @abstractmethod
     def add_message(self, message: BaseMessage) -> None:
         """Add a Message object to the store.
 
         Args:
             message: A BaseMessage object to store.
         """
-        raise NotImplementedError
+        raise NotImplementedError()
 
     @abstractmethod
     def clear(self) -> None: