community: Add deprecation warning for GigaChat integration in langchain-community (#28022)

- **Description:** We have released the
[langchain-gigachat](https://github.com/ai-forever/langchain-gigachat?tab=readme-ov-file)
with new GigaChat integration that support's function/tool calling. This
PR deprecated legacy GigaChat class in community package.

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/langchain_community/chat_models/gigachat.py

@@ -13,6 +13,7 @@ from typing import (
     Type,
 )
 
+from langchain_core._api.deprecation import deprecated
 from langchain_core.callbacks import (
     AsyncCallbackManagerForLLMRun,
     CallbackManagerForLLMRun,
@@ -113,6 +114,11 @@ def _convert_delta_to_message_chunk(
         return default_class(content=content)  # type: ignore[call-arg]
 
 
+@deprecated(
+    since="0.3.5",
+    removal="1.0",
+    alternative_import="langchain_gigachat.GigaChat",
+)
 class GigaChat(_BaseGigaChat, BaseChatModel):
     """`GigaChat` large language models API.