Re-add __add__ functionality for messages (revert #8245) (#8489)

This PR reverts #8245, so `__add__` is defined on base messages.

Resolves issue: https://github.com/langchain-ai/langchain/issues/8472

libs/langchain/langchain/schema/messages.py

@@ -1,12 +1,15 @@
 from __future__ import annotations
 
 from abc import abstractmethod
-from typing import Any, Dict, List, Sequence
+from typing import TYPE_CHECKING, Any, Dict, List, Sequence
 
 from pydantic import Field
 
 from langchain.load.serializable import Serializable
 
+if TYPE_CHECKING:
+    from langchain.prompts.chat import ChatPromptTemplate
+
 
 def get_buffer_string(
     messages: Sequence[BaseMessage], human_prefix: str = "Human", ai_prefix: str = "AI"
@@ -77,6 +80,12 @@ class BaseMessage(Serializable):
         """Whether this class is LangChain serializable."""
         return True
 
+    def __add__(self, other: Any) -> ChatPromptTemplate:
+        from langchain.prompts.chat import ChatPromptTemplate
+
+        prompt = ChatPromptTemplate(messages=[self])
+        return prompt + other
+
 
 class BaseMessageChunk(BaseMessage):
     def _merge_kwargs_dict(
@@ -102,7 +111,7 @@ class BaseMessageChunk(BaseMessage):
                 )
         return merged
 
-    def __add__(self, other: Any) -> BaseMessageChunk:
+    def __add__(self, other: Any) -> BaseMessageChunk:  # type: ignore
         if isinstance(other, BaseMessageChunk):
             # If both are (subclasses of) BaseMessageChunk,
             # concat into a single BaseMessageChunk