Support third-party checkout confirmation

Orders and subscriptions created by untrusted apps are confirmed by the
user on Anikeen ID. confirmationUrl($returnUrl) on Order and
Subscription builds the link with the return URL; checking out such a
draft throws ConfirmationRequiredException carrying the same URL.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Id/Exceptions/ConfirmationRequiredException.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Anikeen\Id\Exceptions;
+
+use Throwable;
+
+/**
+ * Thrown when a third-party app tries to check out an order or subscription itself: the user has
+ * to confirm it on Anikeen ID. Send them to the confirmation URL instead.
+ */
+class ConfirmationRequiredException extends ResourceException
+{
+    public function __construct(string $message, int $code, public readonly ?string $confirmationUrl = null, ?Throwable $previous = null)
+    {
+        parent::__construct($message, $code, $previous);
+    }
+}