App subscriptions: manage a user's subscriptions with the app's own token

- $user->appSubscriptions(): create (idempotency_key), find, all, checkout, revoke, pause,
  resume, update against /v1/client/subscriptions with a client credentials token, so apps don't
  depend on the user's login
- user id from the billable's anikeen_id (AnikeenId::useUserIdField())
- ApiException with status, stable error code, isNotFound/isConflict/isTemporary; network
  errors no longer escape as raw Guzzle exceptions
- rejected app tokens are renewed once; asApp() never changes the shared client's token
- tests (testbench + Guzzle mock) and CI

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

tests/Id/TestCase.php

@@ -0,0 +1,62 @@
+<?php
+
+namespace Anikeen\Id\Tests;
+
+use Anikeen\Id\AnikeenId;
+use Anikeen\Id\Contracts\AppTokenRepository;
+use Anikeen\Id\Providers\AnikeenIdServiceProvider;
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Middleware;
+use GuzzleHttp\Psr7\Response;
+use Orchestra\Testbench\TestCase as Orchestra;
+
+abstract class TestCase extends Orchestra
+{
+    protected MockHandler $http;
+
+    /** @var list<array{request: \Psr\Http\Message\RequestInterface}> */
+    protected array $sent = [];
+
+    protected function getPackageProviders($app): array
+    {
+        return [AnikeenIdServiceProvider::class];
+    }
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        config(['services.anikeen.client_id' => 'app-client', 'services.anikeen.client_secret' => 'secret']);
+
+        $this->http = new MockHandler();
+        $stack = HandlerStack::create($this->http);
+        $stack->push(Middleware::history($this->sent));
+
+        $this->app->instance(AnikeenId::class, (new AnikeenId())->setHttpClient(new Client(['handler' => $stack, 'base_uri' => 'https://id.test/api/'])));
+        $this->app->instance(AppTokenRepository::class, new class implements AppTokenRepository {
+            public int $forgotten = 0;
+
+            public function getAccessToken(): string
+            {
+                return $this->forgotten ? 'fresh-app-token' : 'app-token';
+            }
+
+            public function forgetAccessToken(): void
+            {
+                $this->forgotten++;
+            }
+        });
+    }
+
+    protected function respond(int $status, array $body = []): void
+    {
+        $this->http->append(new Response($status, ['Content-Type' => 'application/json'], json_encode($body)));
+    }
+
+    protected function sentJson(int $index): array
+    {
+        return json_decode((string) $this->sent[$index]['request']->getBody(), true) ?? [];
+    }
+}