community[patch]: GigaChat model selection fix (#20988)

Fixed the error that the model name is never actually put into GigaChat
request payload, always defaulting to `GigaChat-Lite`.

With this fix, model selection through
```python
import os
from langchain.chat_models.gigachat import GigaChat

chat = GigaChat(
    name="GigaChat-Pro", # <- HERE!!!!!
    ...
)
```
should actually work, as intended in
[here](804390ba4b/libs/community/langchain_community/llms/gigachat.py (L36)).

---------

Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/community/langchain_community/chat_models/gigachat.py

@@ -133,6 +133,7 @@ class GigaChat(_BaseGigaChat, BaseChatModel):
         )
 
         payload.functions = kwargs.get("functions", None)
+        payload.model = self.model
 
         if self.profanity_check is not None:
             payload.profanity_check = self.profanity_check