test(e2e): stub the notifications endpoint in the backend-free suite

The bell polls GET /api/v1/notifications from every shell it mounts in,
starting on load, so every stubbed route acquired a failed request. The
hook swallows it and shows an empty bell, but the browser still logs the
request itself, and console-clean.spec counts that: seven routes failed
on "Failed to load resource: 500" pointing at the poll.

Stubbed empty, for the same reason and in the same place as the policy
reconcile above it.

frontend/editor/src/core/tests/helpers/api-stubs.ts

@@ -273,6 +273,14 @@ export async function mockAppApis(
   await page.route("**/api/v1/policies/runs", (route: Route) =>
     route.fulfill({ json: [] }),
   );
+
+  // The notification bell polls GET /api/v1/notifications from every shell it
+  // is mounted in, starting on load. The hook swallows the failure and shows an
+  // empty bell, but the browser still logs the failed request itself, which the
+  // console-hygiene guard counts. Empty list, for the same reason as policies.
+  await page.route("**/api/v1/notifications*", (route: Route) =>
+    route.fulfill({ json: { notifications: [] } }),
+  );
 }
 
 /**