Add checkout sessions and webhook event type constants

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

src/Id/Resources/CheckoutSession.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace Anikeen\Id\Resources;
+
+use Anikeen\Id\Concerns\HasBillable;
+use Anikeen\Id\Concerns\IsCheckoutSession;
+use Throwable;
+
+/**
+ * A checkout session created with the user's access token, see CheckoutSessions.
+ *
+ * @property string $id
+ * @property string $user_id
+ * @property ?string $oauth_client_id
+ * @property string $status open, completed, cancelled or expired
+ * @property ?string $return_url
+ * @property ?object $metadata
+ * @property ?string $confirmation_url
+ * @property list<string> $subscription_ids
+ * @property list<object> $subscriptions
+ * @property ?string $expires_at
+ * @property ?string $completed_at
+ * @property ?string $cancelled_at
+ * @property ?string $created_at
+ */
+class CheckoutSession extends BaseResource
+{
+    use HasBillable;
+    use IsCheckoutSession;
+
+    public const STATUS_OPEN = 'open';
+    public const STATUS_COMPLETED = 'completed';
+    public const STATUS_CANCELLED = 'cancelled';
+    public const STATUS_EXPIRED = 'expired';
+
+    /**
+     * Withdraw an open session: it and its drafts are cancelled.
+     *
+     * @throws Throwable
+     */
+    public function cancel(): self
+    {
+        return (new self(fn () => $this->billable->anikeenId()
+            ->request('POST', sprintf('v1/checkout-sessions/%s/cancel', $this->id))))
+            ->setBillable($this->billable);
+    }
+
+    /**
+     * @throws Throwable
+     */
+    public function refresh(): self
+    {
+        return (new self(fn () => $this->billable->anikeenId()
+            ->request('GET', sprintf('v1/checkout-sessions/%s', $this->id))))
+            ->setBillable($this->billable);
+    }
+}