Admin: Return WP_Error from add_meta() for protected or unauthorized meta keys

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/32565

When a user tried to add a custom field with an underscore-prefixed (protected) key via the Classic Editor's Custom Fields panel, WordPress showed:

"Please provide a custom field value."

This is misleading as the value was present, but the key is protected. The root cause is that add_meta() returned plain false for two distinct failure modes (missing value and protected/unauthorized key), making them indistinguishable to the AJAX handler.

• post.php - add_meta() now returns a WP_Error (code protected_meta) when the key is protected or the user lacks add_post_meta capability. Plain false is kept for "no key/value pair provided" to preserve
  backward compatibility.
• ajax-actions.php - wp_ajax_add_meta() checks is_wp_error() before the existing ! $meta_id fallback so each failure gets the right message. The update path is also updated from wp_die( -1 ) to a translatable human-readable string for consistency.
• tests/phpunit/tests/admin/includesPost.php - the data-provider test covers all 4 authorization and protection scenarios in a single method (admin vs subscriber, protected vs unprotected key).

Testing

npm run test:php -- --filter=Tests_Admin_IncludesPost

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