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/page.tsx

@@ -4,6 +4,7 @@
 
 import { redirect } from 'next/navigation';
 
+/* istanbul ignore next - Next.js server-side redirect covered by e2e tests */
 export default function RootPage() {
   // Redirect to default locale (en)
   redirect('/en');