FIX: mock hierarchical_search in mobile sidebar slide-out test (#38222)

A test I added in 8ffdde5e59 seems to fail
sometimes in CI
`{"type":"error","text":"Unhandled request in test environment:
/categories/hierarchical_search?term= (GET)"}` due to a missing endpoint

this should fix it

frontend/discourse/tests/acceptance/sidebar-user-categories-section-test.js

@@ -1373,6 +1373,11 @@ acceptance(
     });
     needs.mobileView();
     needs.settings({ navigation_menu: "sidebar" });
+    needs.pretender((server, helper) => {
+      server.get("/categories/hierarchical_search", () => {
+        return helper.response({ categories: [] });
+      });
+    });
 
     test("hamburger closes when header dropdown navigates to a new page", async function (assert) {
       await visit("/");