add_action('woocommerce_order_note_added', function($comment_id, $comment) {
    if (empty($comment->comment_post_ID)) return;

    $order = wc_get_order($comment->comment_post_ID);
    if (!$order) return;

    if (stripos($comment->comment_content, 'Payment confirmed via Site B callback') === false) return;

    if ($order->has_status(['processing'])) {
        $order->update_status('completed', 'Auto-completed after Site B callback.');
    }
}, 10, 2);