refactor: Migrate auth handler to pydantic

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: kwonkwonn

src/ai/backend/client/func/user.py

@@ -14,7 +14,7 @@
 from ai.backend.client.session import api_session
 from ai.backend.client.types import set_if_set
 from ai.backend.client.utils import dedent as _d
-from ai.backend.common.dto.manager.auth.field import AuthResponse, parse_auth_response
+from ai.backend.common.dto.manager.auth.types import AuthResponse, parse_auth_response
 
 from .base import BaseFunction, api_function, resolve_fields