Tests: Add unit test coverage for WP_Admin_Notice_Collector

[huzaifaalmesbah]

Description

Adds a test class for WP_Admin_Notice_Collector, which was introduced for #50486 without tests. The tests cover capture_notice() (storing a notice under its ID, returning the markup unchanged, ignoring notices without visible text, counting a '0' message, and counting each notice ID once), get_notices(), get_notice_count() and reset(), init() (collecting notices generated by wp_get_admin_notice(), not adding the title placeholder outside the admin, and hooking only once), add_title_placeholder(), and inject_notice_count() for zero, one and several notices and for output without a placeholder.

The collector's static state is saved in set_up() and restored in tear_down(). The output-buffer branch of init() in the admin is not covered.

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