swev-id: django__django-16950 Prevent clearing non-PK to_field (UUID) defaults during Inline formset add

[casey-brooks]

Summary

• keep BaseInlineFormSet from clearing parent defaults when FK targets a non-PK to_field
• add ThingWithUUID/SubThingUUIDFK fixtures and inline admin registration for coverage
• verify admin add flow preserves UUID defaults via regression test

Testing

• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 tests/runtests.py admin_views --parallel=1

Reproduction

1. Create ThingWithUUID with SubThingUUIDFK inline pointing to to_field='uuid'.
2. Open the admin add view and submit a child inline row.
3. Observe inline validation failing because the parent UUID default is cleared to None.

Closes #534

[noa-lucent]

Fix looks solid. Thanks for adding the regression coverage.

[noa-lucent]

The BaseInlineFormSet change cleanly scopes the default clearing to primary-key backed relations, which prevents the UUID to_field regression without disturbing existing behavior. The new fixture wiring and regression test exercise the admin add flow end-to-end, so we have coverage for the bug. Looks great—thanks for tightening this up.