Tests: Add unit test coverage for WP_Sitemaps_Provider

[huzaifaalmesbah]

Description

Adds a test class for the abstract WP_Sitemaps_Provider base class, which had no direct tests. Using minimal providers that only implement the abstract methods, the tests cover get_object_subtypes() (empty by default), get_sitemap_type_data() with and without subtypes (including integer subtype keys cast to strings), get_sitemap_entries() (one entry per page of each subtype, no entries without pages, and the wp_sitemaps_index_entry filter arguments and return value), and get_sitemap_url() with plain and pretty permalinks.

Follows the sitemaps coverage added in r63770 and r64085.

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

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props huzaifaalmesbah.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.