Tests: Add unit test coverage for Walker_Category_Checklist

[huzaifaalmesbah]

Description

Adds a test class for Walker_Category_Checklist, which was only exercised indirectly through wp_terms_checklist(). The tests call the walker directly. For list-only mode they assert the literal list item markup for selected and popular categories, including IDs passed as strings. For the checkbox markup, where the element IDs are unique per call, they read the attributes with the HTML Tag Processor: the checkbox name for the category and other taxonomies, the ID prefix, distinct IDs for the list item and the checkbox, and the checked, disabled and popular states. They also cover the level and element wrappers, the the_category filter arguments and escaping of its result in both modes, and nesting of child categories through walk().

Category objects are built in memory, so the tests do not touch the database.

This is a test-only change; no production code is modified.

Testing Instructions

1. Run the tests on single site:
   npm run test:php -- --filter 'Tests_Admin_Walker_Category_Checklist'
2. Run them on multisite:
   npm run test:php -- --filter 'Tests_Admin_Walker_Category_Checklist' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'Tests_Admin_Walker_Category_Checklist' --order-by=random
4. Run the whole group to confirm nothing else is affected:
   npm run test:php -- --group admin
5. Run PHPCS on the test file:
   node ./tools/local-env/scripts/docker.js run --rm php ./vendor/bin/phpcs tests/phpunit/tests/admin/walkerCategoryChecklist.php

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

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Brainstorming edge-case scenarios and drafting test boilerplate; test architecture, assertions, and local PHPUnit verification were written and validated by me.

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