Media: Ignore the global post in media_handle_sideload() when no post ID is passed. - #14194

When media_handle_sideload() is called without a post ID, get_post( 0 ) falls back to the global post, so the file is stored in that post's year/month upload folder instead of the current one. This only looks up the post when a post ID is passed. It refreshes 43119.2.diff with new unit tests that set a global post.

Testing instructions

1. Create a post dated in the past (for example January 2018).
2. With that post set as the global $post, sideload a file without a post ID, for example with wp eval:

   require_once ABSPATH . 'wp-admin/includes/media.php';
   require_once ABSPATH . 'wp-admin/includes/file.php';
   require_once ABSPATH . 'wp-admin/includes/image.php';
   $GLOBALS['post'] = get_post( <old post ID> );
   $tmp = wp_tempnam( 'logo.png' );
   copy( ABSPATH . 'wp-includes/images/w-logo-blue.png', $tmp );
   echo get_attached_file( media_handle_sideload( array( 'name' => 'logo.png', 'tmp_name' => $tmp ), 0 ) );

3. Before: the file is saved in uploads/2018/01/. After: it is saved in the current month's folder.
4. Run npm run test:php -- --filter Tests_Media_mediaHandleSideload.

Trac ticket: https://core.trac.wordpress.org/ticket/43119

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Refreshing the existing patch, writing the unit tests and verifying with WP-CLI; reviewed and tested by me.

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.