Fix invalid menu-item-parent

[azaozz]

Follow up to #56926.

There seems to be a bug in the Customizer that may set $menu_item->menu_item_parent to be the same as $menu_item->ID. Steps to reproduce: https://core.trac.wordpress.org/ticket/56926#comment:21. There also seems to be a related JS error:

Uncaught TypeError: parentControl is undefined
    .../wp-admin/js/customize-nav-menus.js?ver=6.2-alpha-54642-src:2330

[mukesh27]

Thanks @azaozz for the ticket!

This ticket was discussed during the recent bug scrub. It looks like it's unlikely that work will be done on this during the 6.2 cycle.

Is this still possible to land in 6.2, or should it be moved to Future Release for now?

[costdev]

This ticket was discussed in the bug scrub and it was agreed to move this ticket to 6.3.

[oglekler]

Due to lack of activity and a patch I am moving this ticket to the 6.4 milestone.

[oglekler]

This ticket was discussed during a bug scrub. Last chance for this ticket :)

Because we already moved it a couple of times, the next step for it is Future releases.

Add props to @mukesh27

[oglekler]

We have no patch and solving the issue can be tricky, so I am moving this to 6.5.

[audrasjb]

As per today's bug scrub, we decided to move this ticket to Future Release to give it more time to get a patch.

Trac ticket: https://core.trac.wordpress.org/ticket/57170

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See ​GitHub Pull Requests for Code Review in the Core Handbook for more details.

### Before:

​https://github.com/user-attachments/assets/4c033e42-9553-4295-b949-90e2797bae7f

### After:

​https://github.com/user-attachments/assets/73d5d9e6-7898-464f-b2f3-9525408e62a3

[lakshyajeet]

Before the fix (reproducing the error)

[lakshyajeet]

After the Fix

[oglekler]

@azaozz please check the patch. It looks like a strait forward solution.

Looks good here. Thanks for the videos :)