Tests: Add unit test coverage for cat_is_ancestor_of()

[rejaulalomkhan]

Adds unit test coverage for cat_is_ancestor_of() in src/wp-includes/category.php.

Test scenarios covered:

• Direct parent category relationships (parent -> child, grandparent -> parent).
• Indirect ancestor relationships (grandparent -> child).
• Non-ancestor relationships (reverse hierarchy, unrelated categories).
• Comparing a category to itself (cat1 === cat2 returns false).
• Parameter combinations: category IDs, WP_Term objects, and mixed ID/object inputs via @dataProvider.
• Non-existent category IDs.
• Invalid or empty inputs (0, '', null, false) via @dataProvider.
• Terms from a different taxonomy (post_tag).

Verification:

• Single-site PHPUnit: OK (16 tests, 23 assertions)
• Multisite PHPUnit: OK (16 tests, 23 assertions)
• Randomized order PHPUnit: OK (16 tests, 23 assertions)
• PHPCS (WordPress-Core): Clean (0 errors, 0 warnings)
• Mutation testing: Verified intentional test failure when return value is inverted.

Trac ticket: https://core.trac.wordpress.org/ticket/65819

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Use of AI Tools: Antigravity (Gemini 2.5) was used to assist in test case ideation, code scaffolding, and formatting to WordPress core standards. All assertions and test outcomes were verified locally through PHPUnit and PHPCS.

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity
Model(s): Gemini 2.5
Used for: Writing PHPUnit unit test coverage for cat_is_ancestor_of(), running test assertions, PHPCS, and quality gate checks. All code was reviewed and verified.

[github-actions]

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Props rejaulalomkhan, lancewillett.

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[Copilot]

🟡 Changes recommended

The replacement taxonomy checks lose existing regression coverage for the category-only restriction.

1 open issue

• Restore hierarchical category pair to preserve taxonomy restriction coverage · New

What changed in this PR

Expands and reorganizes PHPUnit coverage for cat_is_ancestor_of() without changing production code.

Changes:

• Splits hierarchy checks into focused tests.
• Adds data providers for parameter types and invalid inputs.
• Adds missing-category and cross-taxonomy cases.

File	Description
tests/​phpunit/​tests/​category/​catIsAncestorOf.php	Refactors fixtures and expands category ancestry tests.

_{Review effort: Balanced}

[lancewillett]

Please retain the existing cross-taxonomy regression test. The replacement checks weaken that protection even though they pass.

Reviewed with the unit-tests audit and the fresh Copilot review.

_{AI review · gpt-6}

[lancewillett]

Please restore test_returns_false_for_term_ids_from_another_taxonomy() with a real parent–child pair in another hierarchical taxonomy. These new tag/category pairs are unrelated, so they still return false if taxonomy lookup is accidentally unrestricted. The deleted related-term fixture would detect that regression.

[rejaulalomkhan]

@lancewillett Thanks for the review! Addressed in bd939b7:

• Restored test_returns_false_for_term_ids_from_another_taxonomy() with a real parent–child pair in a hierarchical wptests_tax taxonomy, next to the tag checks.
• Checked it: when cat_is_ancestor_of() passes an empty taxonomy to term_is_ancestor_of(), only this test fails; the tag/category checks still pass.