Tests: Improve unit test coverage for WP_Sitemaps_Users

[huzaifaalmesbah]

Description

Expands the WP_Sitemaps_Users tests, which only covered two get_url_list() cases. The new tests cover pagination of the URL list, authors of public custom post types, users without published posts, the wp_sitemaps_users_pre_url_list, wp_sitemaps_users_entry and wp_sitemaps_users_query_args filters (arguments received and return value used), and get_max_num_pages(), which had no coverage, including its wp_sitemaps_users_pre_max_num_pages short-circuit. Expected URLs are asserted as literal strings.

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 'wpSitemapsUsers'
2. Run them on multisite:
   npm run test:php -- --filter 'wpSitemapsUsers' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'wpSitemapsUsers' --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/wpSitemapsUsers.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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