fix(astrolabe): delete stale tokens when refresh fails

- Delete stored token when refresh callback fails or returns null
- Delete stored token when expired with no refresh callback available
- Fix test namespaces (Service → OCA\Astrolabe\Tests\Unit\Service)
- Update tests to verify token deletion on refresh failure

Prevents repeated refresh attempts with stale tokens that will never
succeed, improving error handling and reducing unnecessary API calls.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

third_party/astrolabe/tests/unit/Service/IdpTokenRefresherTest.php

@@ -2,7 +2,7 @@
 
 declare(strict_types=1);
 
-namespace Service;
+namespace OCA\Astrolabe\Tests\Unit\Service;
 
 use OCA\Astrolabe\Service\IdpTokenRefresher;
 use OCA\Astrolabe\Service\McpServerClient;