Repair wp/5.8 branch Actions: Add wpop_keywords to the pattern directory test fixtures - #84458

Part of #82935.

What?

Adds "wpop_keywords": "" to the meta object of every pattern in the four pattern directory test fixtures in phpunit/fixtures/rest-api/pattern-directory/ (12 patterns), test fixtures only.

Why?

The PHP 8.0 legs of unit-test.yml (single site and multisite) report two errors:

1) WP_REST_Pattern_Directory_Controller_Test::test_prepare_item
Undefined property: stdClass::$wpop_keywords

2) WP_REST_Pattern_Directory_Controller_Test::test_prepare_item_search
Undefined property: stdClass::$wpop_keywords

WordPress 5.8 ships WP_REST_Pattern_Directory_Controller, so the class_exists() guard in lib/load.php skips Gutenberg's copy and these tests run against core's class. Core's prepare_item_for_response() reads $raw_pattern->meta->wpop_keywords. Gutenberg's fixtures were written for its own copy of the controller, which reads keyword_slugs, so they don't have that field. On PHP 7 the missing property is a notice that doesn't fail these jobs; on PHP 8 it is a warning, which PHPUnit converts to an error.

How?

Same change as wp/5.9's #84423; the four fixtures are now byte-identical to wp/5.9's.

wp/5.9's #84423 also removed the assertContains( 'core', … ) loop from test_get_items_by_keyword(). That isn't needed here: on this branch the test is skipped with markTestSkipped() (along with most of this class's tests, since bad9073 (#32228)), so only test_prepare_item() and test_prepare_item_search() load these fixtures.

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This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.

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