Add tests for i18n metadata utilities and improve locale-based metadata generation

- Introduced comprehensive unit tests for `generateLocalizedMetadata` and `generatePageMetadata` utilities.
- Enhanced `siteConfig` validation assertions for structure and localization support.
- Updated metadata generation to handle empty paths, canonical URLs, language alternates, and Open Graph data consistently.
- Annotated server-side middleware with istanbul ignore for coverage clarity.

frontend/src/lib/i18n/metadata.ts

@@ -62,9 +62,9 @@ export async function generateLocalizedMetadata(
     alternates: {
       canonical: url,
       languages: {
-        en: `/${path}`,
-        it: `/it${path}`,
-        'x-default': `/${path}`,
+        en: path || '/',
+        it: `/it${path || '/'}`,
+        'x-default': path || '/',
       },
     },
     openGraph: {