core(minor): Allow explicit types for ChatMessageHistory adds (#14967)

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## Description
Changes the behavior of `add_user_message` and `add_ai_message` to allow
for messages of those types to be passed in. Currently, if you want to
use the `add_user_message` or `add_ai_message` methods, you have to pass
in a string. For `add_message` on `ChatMessageHistory`, however, you
have to pass a `BaseMessage`. This behavior seems a bit inconsistent.
Personally, I'd love to be able to be explicit that I want to
`add_user_message` and pass in a `HumanMessage` without having to grab
the `content` attribute. This PR allows `add_user_message` to accept
`HumanMessage`s or `str`s and `add_ai_message` to accept `AIMessage`s or
`str`s to add that functionality and ensure backwards compatibility.

## Issue
* None

## Dependencies
* None

## Tag maintainer
@hinthornw
@baskaryan 

## Note
`make test` results in `make: *** No rule to make target 'test'.  Stop.`

libs/core/langchain_core/chat_history.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
-from typing import List
+from typing import List, Union
 
 from langchain_core.messages import (
     AIMessage,
@@ -42,21 +42,27 @@ class BaseChatMessageHistory(ABC):
     messages: List[BaseMessage]
     """A list of Messages stored in-memory."""
 
-    def add_user_message(self, message: str) -> None:
+    def add_user_message(self, message: Union[HumanMessage, str]) -> None:
         """Convenience method for adding a human message string to the store.
 
         Args:
-            message: The string contents of a human message.
+            message: The human message to add
         """
-        self.add_message(HumanMessage(content=message))
+        if isinstance(message, HumanMessage):
+            self.add_message(message)
+        else:
+            self.add_message(HumanMessage(content=message))
 
-    def add_ai_message(self, message: str) -> None:
+    def add_ai_message(self, message: Union[AIMessage, str]) -> None:
         """Convenience method for adding an AI message string to the store.
 
         Args:
-            message: The string contents of an AI message.
+            message: The AI message to add.
         """
-        self.add_message(AIMessage(content=message))
+        if isinstance(message, AIMessage):
+            self.add_message(message)
+        else:
+            self.add_message(AIMessage(content=message))
 
     @abstractmethod
     def add_message(self, message: BaseMessage) -> None: