Fix: Import Optional from typing in config.py

- Added rom typing import Optional to resolve NameError: name 'Optional' is not defined when llm_base_url is defined with type hint Optional[str].

config.py

@@ -9,6 +9,7 @@ from pydantic_settings import BaseSettings
 from pydantic import Field, field_validator
 from functools import lru_cache
 import os
+from typing import Optional
 
 
 class Settings(BaseSettings):