Detail page menu parent becomes unfixable - tiny edge case

[swirtSJW]

Describe the defect

This defect only shows up in one edge case that is very unlikely to occur in the wild. Here are the requirements to meet the edge case

1. VAMC editor

2. Editing an existing page that is already published and exists directly under "Work with us" (an intentionally un-selectable parent for non-admins) Example: /lebanon-health-care/work-with-us/jobs-and-careers

3. Change the parent from "work with us" to "Select a value" (essentially making it empty)

4. Attempt to save the node either as draft of published

5. The save will get rejected because parent can not be empty.

6. Try to set the parent back to "Work with us" and you will see that it is not a choose-able option (which most of the time is what we want).

7. The editor is left with two options.

   1. Refresh the page (losing anyother changes they might have made.
   2. Move it somewhere it doesn't belong and contact a content admin to move it back under the Work with us parent.

AC / Expected behavior

• [ ] If a user edits a detail page that is a child of "work with us" and mistakenly decides to give the menu no parent, and attempts to save, they should be allowed to choose "work with us" as a parent because that was where it was originally.
  • This should not be specific to work with us but should apply to any existing parent that is a locked with no children allowed parent.

Screenshots

If applicable, add screenshots to help explain your problem.

Additional context

• The defect was discovered in the review of this PR for this issue #17018 in this comment
• This could likely be resolved by getting the original parent from the $form_state and then making sure the original parent is an unlocked choice.

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• [x] Issue type (red) (defaults to "Defect")
• [ ] CMS subsystem (green)
• [ ] CMS practice area (blue)
• [x] CMS workstream (orange) (not needed for bug tickets)
• [ ] CMS-supported product (black)

Team

Please check the team(s) that will do this work.

• [ ] CMS Team
• [ ] Public Websites
• [x] Facilities
• [ ] User support
• [ ] Accelerated Publishing

[swirtSJW]

The easiest place to resolve this, is probably here MenuReductionService::applyVamcMenuRulesForDetailPage()

Inside the big loop if a parent option == the existing parent from the default revision, then do not lock it.