Fix menu item selection persistence across pagination

[ekamran]

Trac ticket: https://core.trac.wordpress.org/ticket/65348

Summary

This fixes an issue in the Menus screen where selected menu items are lost when moving between paginated meta box pages before clicking "Add to Menu".

The patch stores selected menu item data per meta box before AJAX pagination replaces the checklist markup, restores checked state when returning to a page, and clears stored selections when switching tabs or after adding items.

Testing

Tested locally:

• Pages: selected items on page 1 and page 3, then added them together.
• Pages: returned to page 1, verified checks were restored, unchecked one item, and confirmed it was not added.
• Select All: selected current page items, paginated, selected another item, and added all expected items.
• Categories: selected items across taxonomy pagination and added all expected items.
• Tab switching: confirmed cached selections are cleared.
• Custom Links: confirmed custom links still add normally.
• Real AJAX add-menu-item request: confirmed selected items from multiple pages are added.
• Ran npm run grunt -- jshint:core --file=src/js/_enqueues/lib/nav-menu.js.
• Ran git diff --check.

Also checked compatibility with the 6.9.4 tag:

• Patch applies cleanly to 6.9.4.
• jshint:core passes on the patched 6.9.4 file.
• Same browser regression checks passed using the patched 6.9.4 nav-menu.js.

[github-actions]

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[github-actions]

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[github-actions]

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[ekamran]

Added QUnit coverage for this patch, in tests/qunit/wp-admin/js/nav-menu.js. It exercises the pagination, select all, tab switch and add to menu paths by firing real clicks, so the wiring is covered and not just the helpers.

While writing it I found the fix missed one case. Rows that do not map to a real object, such as Home and post type archives, get the negative $_nav_menu_placeholder as their object ID. That counter restarts every request, so those rows came back under a different ID and lost their tick. On a default install with no static front page the Home row is always present, so this was easy to hit.

That is a separate commit (e76e2f3) so it can be dropped if you would rather handle it in its own ticket.