Tests: Add unit test coverage for Walker_CategoryDropdown

[huzaifaalmesbah]

Description

Adds a test class for Walker_CategoryDropdown, which was only exercised indirectly through wp_dropdown_categories(). The tests call the walker directly and assert the literal <option> markup: the depth class and padding, selection by integer ID, string ID and slug, falsy selected values, the value_field argument and its fallback to the term ID, the formatted post count, escaping of the option value, appending to existing output, the list_cats filter arguments and return value, 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_Category_Walker_CategoryDropdown'
2. Run them on multisite:
   npm run test:php -- --filter 'Tests_Category_Walker_CategoryDropdown' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'Tests_Category_Walker_CategoryDropdown' --order-by=random
4. Run the whole group to confirm nothing else is affected:
   npm run test:php -- --group category
5. Run PHPCS on the test file:
   node ./tools/local-env/scripts/docker.js run --rm php ./vendor/bin/phpcs tests/phpunit/tests/category/walkerCategoryDropdown.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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