List Tables: Prevent Filter button submission and show admin notice when no filters are selected

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/31634

Adds client-side validation to the Filter button in WP_List_Table admin screens (Posts, Comments, Media, Links, Sites). When the Filter button is clicked with no meaningful filter criteria selected, the form submission is intercepted. A dismissible admin error notice reading "Please select a filter to apply." is injected at the top of the page, and wp.a11y.speak() announces it for screen reader users.

The two new button name identifiers (filter_action, post-query-submit) are registered alongside the existing bulk-action observer IDs in script-loader.php and remain filterable via the bulk_action_observer_ids filter for third-party extensibility.

[github-actions]

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