Fixes #2225

When a ForeignKeyField/OneToOneField is declared with a to_field that points to a non-primary-key column, filtering by a related model instance used the instance's primary key instead of the to_field value.

In Q._get_actual_filter_params, the fk/o2o branch did filter_value = getattr(value, "pk", value), hardcoding .pk. So for a FK with to_field="app_id", Model.filter(fk=instance) generated SQL comparing the <fk>_id column (which stores the to_field value) against instance.pk, targeting the wrong value and, when the types differ (e.g. a UUID pk vs an int to_field), raising type errors or silently returning wrong results.

This resolves the to_field column via field_object.to_field_instance.model_field_name and reads that attribute off the instance. For the default case where to_field is the primary key, to_field_instance.model_field_name is the pk attribute, so getattr returns value.pk exactly as before; raw-value filters (filter(fk=7)) still fall through to the value via getattr's default.

The existing test models already expose this shape: School's primary key is uuid, while Student.school uses to_field="id" (a non-pk unique field). The added regression test in tests/test_relations_with_unique.py creates a School whose id differs from its pk and asserts Student.filter(school=school) finds the student; it fails on the current code (filters by the pk) and passes with this change. The full test_relations and test_relations_with_unique suites pass.