fix: test_unit_routers.py for ci/cd

[SahilJat]

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Changes

Closes #7368

Fixes flaky CI failures caused by inline models.Model subclasses in test_unit_app_routers.py polluting Django's apps registry.

• Removed all inline models.Model subclasses (AnalyticsModel, MyModel, MyModel1, MyModel2) from the parametrised test functions.
• Replaced them with mock.MagicMock() objects that expose _meta.app_label — the only attribute the router inspects.
• Removed the from django.db import models import, replaced with from unittest import mock.
• The allow_migrate test was already clean and required no changes.

Root Cause:
The root cause was that defining a Django model class registers it permanently in the apps registry for the lifetime of the worker process. When tests ran with app_label="app_analytics", the four model classes were permanently registered against the real app_analytics app, causing makemigrations --check to find unmigrated models and Migrator teardown to fail with relation "app_analytics_analyticsmodel" already exists across multiple xdist workers.

How did you test this code?

• Ran the updated test file locally and confirmed all 4 tests pass.
• Confirmed the makemigrations test no longer detects phantom models.
• Confirmed migration tests no longer fail with DuplicateTable errors.

[claude]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (82c5dfb) to head (0b3b81f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7530      +/-   ##
==========================================
- Coverage   98.47%   98.31%   -0.16%     
==========================================
  Files        1400     1400              
  Lines       53036    52909     -127     
==========================================
- Hits        52226    52017     -209     
- Misses        810      892      +82     

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