test: hand the router's navigate out through an effect, not a render-time reassignment

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L9qibz6e1jru1WvD8e9e9E

Author: claude

ui/packages/app-shell/src/layout/__tests__/UnifiedSidebar.areaFollowsRoute-28497.test.tsx

@@ -138,12 +138,21 @@ const serviceArea: NavigationArea = {
   ],
 };
 
-let navigate: NavigateFunction;
+/** The router's navigate, handed out of the tree once it has rendered. */
+const router: { navigate: NavigateFunction | null } = { navigate: null };
 function NavigateProbe() {
-  navigate = useNavigate();
+  const navigate = useNavigate();
+  React.useEffect(() => {
+    router.navigate = navigate;
+  }, [navigate]);
   return null;
 }
 
+function navigate(path: string) {
+  if (!router.navigate) throw new Error('the router has not rendered');
+  void router.navigate(path);
+}
+
 function renderAt(path: string) {
   metadataState = {
     apps: [{ name: 'crm', label: 'CRM', active: true, areas: [salesArea, serviceArea] }],