fix: bind CdnPurgeService stub instead of CdnManager

The type-hinted constructor on CdnPurgeService requires
Plug\Cdn\CdnManager which doesn't exist in test env. Bind
the service itself with a no-op stub.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/TestCase.php

@@ -30,7 +30,12 @@ abstract class TestCase extends BaseTestCase
 
         // Stub external dependencies not available in test environment
         if (! class_exists(\Plug\Cdn\CdnManager::class)) {
-            $app->bind(\Plug\Cdn\CdnManager::class, fn () => new class {});
+            $app->bind(\Core\Mod\Content\Services\CdnPurgeService::class, fn () => new class {
+                public function isEnabled(): bool { return false; }
+                public function purgeContent($content) { return null; }
+                public function purgeUrls(array $urls) { return null; }
+                public function purgeWorkspace(string $uuid) { return null; }
+            });
         }
     }
 }