REST API: Preserve parent when generating unique slug for draft child…

[maulikmakwana2008]

Fixes #64670

When updating a draft hierarchical post via the REST API, post_parent is often
omitted from the request. This caused wp_unique_post_slug() to be called with a
parent ID of 0, resulting in false slug conflicts.

This change falls back to the existing post_parent from $post_before when no
parent is provided, aligning block editor behavior with Quick Edit.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props maulikmakwana2008.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Trac Ticket Missing

This pull request is missing a link to a Trac ticket. For a contribution to be considered, there must be a corresponding ticket in Trac.

To attach a pull request to a Trac ticket, please include the ticket's full URL in your pull request description. More information about contributing to WordPress on GitHub can be found in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.