Tests: Add unit test coverage for Walker_PageDropdown

[huzaifaalmesbah]

Description

Adds a test class for Walker_PageDropdown, which was only exercised indirectly through wp_dropdown_pages(). The tests call the walker directly and assert the literal <option> markup: the depth class and padding, selection by integer and string ID, falsy selected values, the value_field argument and its fallback to the ID (selection always uses the ID), escaping of the title and the option value, the #ID (no title) fallback for an empty title only, appending to existing output, the list_pages filter arguments and that its result is escaped, and nesting of child pages through walk().

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