Honor serialized rollback fixture alongside django_db marker

[AryanRG]

Summary

When a transactional test has both @pytest.mark.django_db and the declared django_db_serialized_rollback fixture, database setup currently uses only the marker's serialized_rollback value. With the default marker value, migrated data is not serialized for restoration even though the fixture enables serialized rollback at execution time. A later transactional test can therefore lose rows inserted by a data migration.

This change makes collection-time database serialization consider either the marker or the declared rollback fixture, matching the existing execution-time behavior. It adds a regression using a real data migration and two successive transactional tests, plus controls for marker and fixture combinations. Dynamic fixture lookup is not changed.

Validation

• Before the fix: four new regression cases failed and two controls passed on both Django 5.2.18 and 6.1.2.
• After the fix: all six cases passed on both versions.
• Django 5.2 adjacent database/fixture suites: 136 passed, 2 skipped.
• Django 6.1 full suite: 246 passed, 2 skipped.
• Ruff lint/format and git diff --check: passed.

Tests were run with SQLite on macOS. PostgreSQL, MySQL, other platforms, and the full hosted matrix remain unverified locally.

AI assistance was used to prepare the implementation and regression tests.