Build/Test Tools: Correct @var annotation in REST users controller tests

[Soean]

What

Fixes an incorrect inline @var annotation in WP_Test_REST_Users_Controller::test_get_items_only_fetches_ids_for_head_requests().

The $query variable comes from the pre_user_query filter args and is a WP_User_Query instance, the assertInstanceOf( WP_User_Query::class, ... ) on the line above asserts exactly that. The docblock incorrectly declared it as WP_User.

Why

WP_User has no query_vars property, so the wrong annotation causes static analysis tools (PHPStan) and IDEs to report false errors on the following $query->query_vars accesses, and it misleads anyone reading the test

Trac ticket: https://core.trac.wordpress.org/ticket/64894

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