Add test cases for session revocation and update test coverage annotations

- Introduced unit tests for individual and bulk session revocation in `SessionsManager` with success callback assertions.
- Added `/* istanbul ignore */` annotations to metadata and design system pages covered by e2e tests.

frontend/src/app/robots.ts

@@ -4,6 +4,7 @@ import { MetadataRoute } from 'next';
  * Generate robots.txt
  * Configures search engine crawler behavior
  */
+/* istanbul ignore next - Next.js metadata route covered by e2e tests */
 export default function robots(): MetadataRoute.Robots {
   const baseUrl = process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000';