[30691] Fix: Reset attachment post_parent when featured image is removed

[Vedanshmini26]

trac: https://core.trac.wordpress.org/ticket/30691

Problem
When a featured image is removed from a post, WordPress only deletes the _thumbnail_id meta field but doesn't reset the attachment's post_parent field back to 0. This creates orphaned relationships where attachments still reference deleted parent posts, causing incorrect results when using functions like get_children() and get_posts().

For Example:
Set image ID 123 as the featured image for post ID 456
Remove the featured image from post 456
Image 123 still has post_parent = 456 in wp_posts table
get_children(456) incorrectly returns image 123 as a child

Root Cause
The delete_post_thumbnail() function in wp-includes/post.php only removes the post meta. Additionally, set_post_thumbnail() doesn't properly establish the post_parent relationship when setting a featured image.

Solution
This PR enhances both functions to properly manage the post_parent relationship:

1. delete_post_thumbnail():
   Retrieves the current thumbnail ID before deletion
   Removes the _thumbnail_id meta field
   Resets the attachment's post_parent to 0 if it was set to the current post

2. set_post_thumbnail():
   Sets the _thumbnail_id meta field
   Updates the attachment's post_parent to establish a proper relationship

[github-actions]

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Props sheldorofazeroth.

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[github-actions]

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