fix(medcat-den): Fix rigid model card format requirement

[mart-r]

As of MedCAT v2.5 the model card defines new keys:

        'Pipeline Description': {"core": dict[str, {"name": str, "provider": str}], "addons": list[{"name": str, "provider": str}]},
        'Required Plugins': list[{"name": str, "provides": list[tuple[str, str]], "author": str | None, "url": str | None}]

MedCAT-den requires the ModelCard type in its ModelInfo model creating a strict dependence on the format. However, older models (and back ends runnning older versions of MedCAT) won't have these fields. So the model verification tends to fail.

This PR adds a field validator that adds empty placeholders for these new fields (if they're missing) when creating the model.

[tomolopolis]

Task linked: CU-869bt0mb4 Fix rigid dependence on model card format

[tomolopolis]

lgtm