Tests: Add unit test coverage for WP_Metadata_Lazyloader

[huzaifaalmesbah]

Description

Adds a test class for WP_Metadata_Lazyloader, which was only exercised indirectly through the term and comment meta lazy-loading functions. Using a fresh instance, the tests cover queue_objects() and reset_queue() for each supported object type and for invalid types, the metadata filter that is hooked and unhooked, the metadata_lazyloader_queued_objects action arguments, and lazyload_meta_callback(): it primes the cache of every queued object plus the requested one, returns the filtered value unchanged, empties the queue, and does nothing for types without queued objects. The deprecated lazyload_term_meta() and lazyload_comment_meta() methods are covered as well.

Metadata caches primed by fixture creation are cleared in set_up() so each assertion observes the lazy-loader itself.

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