Fix flaky CT tests

[amureki]

Describe the change
Finally figured (hopefully) out where flaky ContentType tests are coming from.
pytest-dev/pytest-django#1156

Adding a fixture to clear the cache between the tests.

PR Checklist

• Change is covered with tests
• CHANGELOG.md is updated if needed

Summary by CodeRabbit

• Tests
  • Improved test stability by fixing cache-related issues that could cause intermittent test failures, particularly on PostgreSQL systems.

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[coderabbitai]

📝 Walkthrough

Walkthrough

A test fixture was added to clear the ContentType cache between test executions, preventing stale cache entries from rolled-back transactions and avoiding foreign key violations on PostgreSQL.

Changes

Cohort / File(s)	Summary
Test fixture for cache management tests/test_filling_fields.py	Added clear_content_type_cache autouse fixture within TestFillingGenericForeignKeyField class that clears ContentType cache between tests to prevent FK violations from transaction rollbacks

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A cache that lingers? Not on our watch!
With fixtures so clever, we clear every blotch,
Transactions may rollback, but we've got the cure,
ContentType chaos? We'll keep tests pure! 🧹✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix flaky CT tests' directly addresses the main purpose of the changeset - fixing flaky ContentType (CT) tests by adding a cache-clearing fixture.

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✨ Finishing touches

• 📝 Generate docstrings

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