♻️(backend) refactor ApplicationViewSet to use a basic ViewSet

This endpoint only exposes a custom action for token generation and does not
rely on serializers or querysets. Using ViewSet is more appropriate here, as
it provides routing without enforcing standard CRUD patterns or requiring a
serializer_class.

This removes unnecessary constraints and avoids warnings related to missing
serializer configuration, while better reflecting the actual responsibility of
this view.

I noticed this bug from Sentry issue 241308

src/backend/core/external_api/viewsets.py

@@ -28,7 +28,7 @@ from . import authentication, permissions, serializers
 logger = getLogger(__name__)
 
 
-class ApplicationViewSet(viewsets.GenericViewSet):
+class ApplicationViewSet(viewsets.ViewSet):
     """API endpoints for application authentication and token generation."""
 
     @decorators.action(