Tests: Add unit test coverage for the_modified_author()

[rejaulalomkhan]

Adds unit test coverage for the_modified_author() in src/wp-includes/author-template.php.

Test scenarios covered:

• Displaying the display name of the editor who last edited the post via _edit_last.
• Displaying the author's display name when the author is also the last editor.
• Outputting nothing when the post has no _edit_last post meta.
• Outputting nothing when _edit_last points to a non-existent or deleted user ID.
• Outputting nothing when no global $post exists.
• Modifying displayed output via the the_modified_author filter hook.
• Respecting various custom display names (ASCII, Unicode, HTML entities, single words) via @dataProvider.

Verification:

• Single-site PHPUnit: OK (10 tests, 10 assertions)
• Multisite PHPUnit: OK (10 tests, 10 assertions)
• Randomized order PHPUnit: OK (10 tests, 10 assertions)
• Related getTheModifiedAuthor test suite: OK (5 tests, 9 assertions)
• PHPCS (WordPress-Core): Clean (0 errors, 0 warnings)
• Mutation testing: Verified 7 test failures when the echo statement is omitted.

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.

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[Copilot]

🟢 Approval recommended

The tests match existing behavior and test isolation mechanisms, with no blocking issues identified.

No open issues 🎉

What changed in this PR

Adds unit coverage for the_modified_author() without changing production behavior.

Changes:

• Tests editor and author display names, missing metadata, nonexistent users, and absent global posts.
• Verifies filter customization and varied display names.

File	Description
tests/​phpunit/​tests/​user/​theModifiedAuthor.php	Adds fixtures and output assertions for ten test cases.

_{Review effort: Balanced}

[lancewillett]

The literal output checks distinguish the editor from the original author and cover filtered output and display names. Optional cleanup: trim wrapper cases already covered by get_the_modified_author() while retaining the exact-output and escaping-sensitive cases.

Unit-tests audit and Copilot review found no blockers. PHPCS passed. Each class passed separately, and the combined batch passed randomized single-site and multisite runs: 63 tests, 95 assertions each (seeds 65829 and 65830). CI is green on the reviewed commit.

_{AI review · gpt-6}

[lancewillett]

PR #14456 landed in https://core.trac.wordpress.org/changeset/64548