Split the polymorphic field tests by layer #952

Proposed change

Split tests/test_polymorphic_fields.py (about 2,300 lines) by layer, into the packages that already hold each layer's tests:

• tests/api/test_polymorphic_fields.py: PolymorphicFieldAPITest, which covers creating polymorphic fields and reading and writing their values over REST.
• tests/test_polymorphic_views.py: PolymorphicFieldUITest, which covers the edit, bulk edit, detail and delete pages and their forms.
• tests/models/test_polymorphic_fields.py: the model-level classes. These cover through-model registration, deleting referenced objects, cycle detection, reverse descriptors, get_models() re-entrancy and cache registry consistency.

The _grant_perm helper, used by the API and UI tests, moves to tests/base.py. Tests move unchanged.

Justification

One module mixes REST, UI and model tests, so polymorphic coverage for each layer is hard to find next to that layer's other tests. The NPL-1248 test audit proposed splitting it by layer.