Tests: Add unit test coverage for add_allowed_options(), remove_allowed_options(), and option_update_filter()

[huzaifaalmesbah]

Description

Adds tests for add_allowed_options(), remove_allowed_options() and option_update_filter(), which had no coverage. The tests cover adding options to existing and new pages, duplicates, falsy and loosely equal option names (the functions compare strictly), an existing page value that is not an array, removing options while keeping the remaining array keys, pages and options that do not exist, updating the $allowed_options global when no options are passed and leaving it alone when they are, and option_update_filter() with array and non-array values of the $new_allowed_options global.

The $allowed_options and $new_allowed_options globals are saved in set_up() and restored in tear_down().

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_Admin_Includes_Plugin_AllowedOptions'
2. Run them on multisite:
   npm run test:php -- --filter 'Tests_Admin_Includes_Plugin_AllowedOptions' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'Tests_Admin_Includes_Plugin_AllowedOptions' --order-by=random
4. Run the whole group to confirm nothing else is affected:
   npm run test:php -- --group plugins
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/plugin/allowedOptions.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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