Tests: Add unit test coverage for is_theme_paused(), wp_get_theme_error(), and resume_theme()

[huzaifaalmesbah]

Description

Adds tests for is_theme_paused(), wp_get_theme_error() and resume_theme() in wp-admin/includes/theme.php, which had no coverage. The tests cover the unset global, active stylesheet and template themes, inactive themes, empty and null stored errors, removing a stored error in recovery mode, the error returned outside recovery mode, and the redirect branch of resume_theme(): the error nonce added to the redirect URL and loading of the theme's functions.php file in sandbox scraping mode.

This mirrors the plugin tests in #14156.

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

Testing Instructions

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