Use wp_handle_upload outside of a POST

There is no uploading occurring here, but you did describe sideloading.

So use media_handle_sideload instead, e.g.

$file_array = [
    'name'     => 'test.jpg',
    'tmp_name' => '/path/to/test.jpg'
];

$post_id = 0;
$attachment_id = media_handle_sideload( $file_array, $post_id );
 
if ( is_wp_error( $attachment_id ) ) {
    // ... it failed
}

• https://developer.wordpress.org/reference/functions/media_handle_sideload/
• based on https://developer.wordpress.org/reference/functions/media_handle_sideload/#comment-983

There are other functions that build on this, e.g. media_sideload_image which takes the URL of an image, downloads it, and runs it through the sideload functions,