Add checkout sessions and webhook event type constants

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

src/Id/Resources/AppCheckoutSessions.php

@@ -0,0 +1,79 @@
+<?php
+
+namespace Anikeen\Id\Resources;
+
+use Anikeen\Id\Contracts\Billable;
+use Anikeen\Id\Exceptions\ApiException;
+use Anikeen\Id\Support\AppRequest;
+use Illuminate\Database\Eloquent\Model;
+use InvalidArgumentException;
+
+/**
+ * Checkout sessions created with the app's own token (trusted apps, scope `subscriptions:client`):
+ * several subscriptions of one user that the user confirms together on Anikeen ID, instead of the
+ * app checking them out itself. For one user ($user->appCheckoutSessions()) or, to look sessions
+ * up e.g. from a webhook, for all users (app(AnikeenId::class)->appCheckoutSessions()).
+ * Nothing is requested until a method is called.
+ */
+class AppCheckoutSessions
+{
+    public function __construct(protected Billable|Model|null $billable = null)
+    {
+    }
+
+    /**
+     * Open a checkout session with new subscriptions (their attributes, like
+     * AppSubscriptions::create()) and/or existing drafts of the app (`['id' => …]`).
+     *
+     * @param array{
+     *     subscriptions: list<array{id: string}|array{
+     *         group?: null|string,
+     *         name: string,
+     *         description?: null|string,
+     *         unit: string,
+     *         price: float|string,
+     *         vat_rate?: float,
+     *         payload?: null|array,
+     *         ends_at?: null|string
+     *     }>,
+     *     return_url?: null|string,
+     *     metadata?: null|array,
+     *     idempotency_key?: null|string,
+     *     user_id?: string
+     * } $attributes return_url must share scheme, host and port with one of the app's redirect
+     *               URIs; the user comes back to it with `status` and `checkout_session`. A
+     *               repeated idempotency_key returns the first session instead of a new one.
+     *
+     * @throws ApiException 422 validation (e.g. return_url), 409 "idempotency_key_conflict"
+     */
+    public function create(array $attributes): AppCheckoutSession
+    {
+        if ($this->billable) {
+            $attributes['user_id'] = (new AppSubscriptions($this->billable))->userId();
+        } elseif (empty($attributes['user_id'])) {
+            throw new InvalidArgumentException('A checkout session needs a user: use $user->appCheckoutSessions() or pass user_id.');
+        }
+
+        return new AppCheckoutSession(AppRequest::send('POST', 'v1/client/checkout-sessions', $attributes)->data);
+    }
+
+    /**
+     * A session of the app; null when it doesn't exist (or belongs to another user).
+     *
+     * @throws ApiException
+     */
+    public function find(string $id): ?AppCheckoutSession
+    {
+        try {
+            $session = new AppCheckoutSession(AppRequest::send('GET', sprintf('v1/client/checkout-sessions/%s', $id))->data);
+        } catch (ApiException $e) {
+            if ($e->isNotFound()) {
+                return null;
+            }
+
+            throw $e;
+        }
+
+        return !$this->billable || $session->user_id === (new AppSubscriptions($this->billable))->userId() ? $session : null;
+    }
+}