Pyright complains about django_model_refs not being attributes on the config model class

[stevenbal]

See https://github.com/open-formulieren/open-forms/actions/runs/12082402354/job/33693409873?pr=4822

Explicitly defined attributes work, but type checkers might complain about fields being defined in django_model_refs. I'm not sure how difficult it is to make sure the type hints work for these fields as well