Add checkout sessions and webhook event type constants

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

src/Id/Concerns/IsCheckoutSession.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Anikeen\Id\Concerns;
+
+/**
+ * Reading a checkout session (see AppCheckoutSession and CheckoutSession).
+ */
+trait IsCheckoutSession
+{
+    /**
+     * Where the user confirms (or cancels) all of the session's subscriptions, or null once the
+     * session is no longer open. The user then returns to the session's `return_url` with
+     * `status` (completed, cancelled or expired) and `checkout_session` (the id). Check the status
+     * through the API or the checkout.* webhooks; don't trust the query string.
+     */
+    public function confirmationUrl(): ?string
+    {
+        return $this->confirmation_url ?: null;
+    }
+
+    public function isOpen(): bool
+    {
+        return $this->status === self::STATUS_OPEN;
+    }
+
+    /**
+     * Confirmed: all of its subscriptions are running.
+     */
+    public function isCompleted(): bool
+    {
+        return $this->status === self::STATUS_COMPLETED;
+    }
+
+    /**
+     * Cancelled or expired: all of its drafts were cancelled.
+     */
+    public function isCancelled(): bool
+    {
+        return in_array($this->status, [self::STATUS_CANCELLED, self::STATUS_EXPIRED], true);
+    }
+
+    /**
+     * @return list<string>
+     */
+    public function subscriptionIds(): array
+    {
+        return array_values((array) ($this->subscription_ids ?? []));
+    }
+}