Context

• To improve compatibility with get_permalink.

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where post-sitemap.xml would cause a fatal error when the filter post_link required a WP_Post object. Props to stodorovic.

Relevant technical choices:

• Use sanitize_post instead of casting to int and enforce WP_Post object instead of default stdClass.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

Test instructions for QA when the code is in the RC

1. Add following PHP code:

add_filter( 'post_link', function( $permalink, WP_Post $post ){ 
    return $permalink;
}, 10, 2 );

2. Check post-sitemap.xml. It should work without PHP fatal error.

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• It affects all post_type sitemaps (post, page, CPTs).

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #16308