fix(mypy): use LitellmUserRoles enum instead of raw string in custom_auth_basic

Fixes mypy error: Argument 'user_role' has incompatible type 'str'; expected 'LitellmUserRoles | None'

Co-authored-by: Ishaan Jaff <ishaan-jaff@users.noreply.github.com>

Author: cursoragent

litellm/proxy/example_config_yaml/custom_auth_basic.py

@@ -1,6 +1,6 @@
 from fastapi import Request
 
-from litellm.proxy._types import UserAPIKeyAuth
+from litellm.proxy._types import LitellmUserRoles, UserAPIKeyAuth
 
 
 async def user_api_key_auth(request: Request, api_key: str) -> UserAPIKeyAuth:
@@ -9,7 +9,7 @@ async def user_api_key_auth(request: Request, api_key: str) -> UserAPIKeyAuth:
             api_key="best-api-key-ever",
             user_id="best-user-id-ever",
             team_id="best-team-id-ever",
-            user_role="proxy_admin",
+            user_role=LitellmUserRoles.PROXY_ADMIN,
         )
     except Exception:
         raise Exception