🐛(backend) Fix unreachable exception handler for URLValidator

The exception block was never being executed because
URLValidator raises django.core.exceptions.ValidationError,
not drf.exceptions.ValidationError.

Signed-off-by: Mohamed El Amine BOUKERFA <boukerfa.ma@gmail.com>

Author: maboukerfa

src/backend/core/api/viewsets.py

@@ -2135,7 +2135,7 @@ def cors_proxy(self, request, *args, **kwargs):
         url_validator = URLValidator(schemes=["http", "https"])
         try:
             url_validator(url)
-        except drf.exceptions.ValidationError as e:
+        except ValidationError as e:
             return drf.response.Response(
                 {"detail": str(e)},
                 status=drf.status.HTTP_400_BAD_REQUEST,