Make seat usage available only for top-level groups

Update Groups::SeatUsageController to respond with 404 Not Found if the
group is not top-level group, in order to match API -
https://gitlab.com/gitlab-org/gitlab/-/blob/fa74a193238b237b11577f79298e1203dbc48de9/ee/lib/ee/api/members.rb#L68.

ee/app/controllers/groups/seat_usage_controller.rb

 # frozen_string_literal: true
 
 class Groups::SeatUsageController < Groups::ApplicationController
+  before_action :verify_top_level_group
   before_action :authorize_admin_group_member!
   before_action :verify_namespace_plan_check_enabled
-  before_action :seat_usage_export_enabled?, if: -> { request.format.csv? }
+  before_action :verify_seat_usage_export_enabled, if: -> { request.format.csv? }
 
   layout "group_settings"
 
@@ -36,7 +37,11 @@ class Groups::SeatUsageController < Groups::ApplicationController
     "seat-usage-export-#{Time.current.to_s(:number)}.csv"
   end
 
-  def seat_usage_export_enabled?
+  def verify_top_level_group
+    not_found unless group.root?
+  end
+
+  def verify_seat_usage_export_enabled
     not_found unless Feature.enabled?(:seat_usage_export, group, default_enabled: :yaml)
   end
 end

ee/spec/controllers/groups/seat_usage_controller_spec.rb

@@ -12,8 +12,8 @@ RSpec.describe Groups::SeatUsageController do
       stub_application_setting(check_namespace_plan: true)
     end
 
-    def get_show(format: :html)
-      get :show, params: { group_id: group }, format: format
+    def get_show(format: :html, group_id: group)
+      get :show, params: { group_id: group_id }, format: format
     end
 
     subject { response }
@@ -34,9 +34,26 @@ RSpec.describe Groups::SeatUsageController do
           expect(response).to have_gitlab_http_status(:ok)
           expect(response).to render_template(:show)
         end
+
+        it 'responds with 404 Not Found if the group is not top-level group' do
+          subgroup = create(:group, :private, :nested)
+          subgroup.add_owner(user)
+
+          get_show(group_id: subgroup)
+
+          expect(response).to have_gitlab_http_status(:not_found)
+        end
       end
 
       context 'when csv format' do
+        it 'responds with 404 Not Found if the group is not top-level group' do
+          subgroup = create(:group, :private, :nested)
+          subgroup.add_owner(user)
+          get_show(group_id: subgroup, format: :csv)
+
+          expect(response).to have_gitlab_http_status(:not_found)
+        end
+
         context 'when seat_usage_export feature flag is disabled' do
           before do
             stub_feature_flags(seat_usage_export: false)

ee/spec/frontend/billings/seat_usage/components/subscription_seats_spec.js

@@ -29,7 +29,6 @@ const providedFields = {
   namespaceName: 'Test Group Name',
   namespaceId: '1000',
   seatUsageExportPath: '/groups/test_group/-/seat_usage.csv',
-
 };
 
 const fakeStore = ({ initialState, initialGetters }) =>
@@ -75,7 +74,7 @@ describe('Subscription Seats', () => {
   const findPageHeadingText = () => findPageHeading().find('[data-testid="heading-info-text"]');
   const findPageHeadingBadge = () => findPageHeading().find(GlBadge);
 
-  const findExportButton= () => wrapper.findByTestId('export-button');
+  const findExportButton = () => wrapper.findByTestId('export-button');
 
   const findSearchBox = () => wrapper.findComponent(FilterSortContainerRoot);
   const findPagination = () => wrapper.findComponent(GlPagination);