feat: MRTTestCase for unittest-based Django migration testing

[croc100]

Summary

• Adds MRTTestCase(unittest.TestCase) — drop-in base class for Django teams not using pytest
• assertRollbackSafe() delegates to DjangoRollbackVerifier (full seed+upgrade+downgrade+verify cycle)
• assertDataIntact() verifies both user-created rows AND SmartSeeder rows survive rollback
• old_apps / apps properties expose historical Django app registries for each migration state

Bug fixes in this PR

#	Bug	Fix
1	assertDataIntact only checked SmartSeeder rows, not user ORM data	PK snapshot before seeder; verifies both sets after rollback
2	No recovery if downgrade() raised mid-test	try/except → downgrade_app_zero fallback, then re-raise
3	_historical_apps hardcoded connections["default"]	@classmethod reads cls.db_alias (default: "default"); override for multi-DB
4	__init__.py imported MRTTestCase unconditionally	Wrapped in try/except ImportError — safe for non-Django users

Test plan

• 12 unit tests in tests/test_django_testcase.py — all mocked, no real DB required
• New: test_assert_data_intact_recovery_on_downgrade_failure — verifies downgrade_app_zero is called
• New: test_assert_data_intact_detects_user_row_loss — verifies pre-existing rows are tracked
• Full suite: 434 passed, 17 skipped

Usage

from pytest_mrt.django_testcase import MRTTestCase

class TestRemovePhoneField(MRTTestCase):
    db_url       = "sqlite:///test.db"
    migrate_from = ("myapp", "0009_add_phone")
    migrate_to   = ("myapp", "0010_remove_phone")

    def test_rollback_is_safe(self):
        self.assertRollbackSafe()

    def test_existing_rows_survive(self):
        User = self.old_apps.get_model("myapp", "User")
        User.objects.create(name="Alice", phone="+1-555-0100")
        self.assertDataIntact()   # Alice must still be there after rollback

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 77.45098% with 23 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pytest_mrt/django_testcase.py	77.89%	16 Missing and 5 partials ⚠️
pytest_mrt/__init__.py	71.42%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!