Share JS code for all paths in trials

Namespace select stops working if a customer receives
an error on applying the trial plan. This happens due to
redirect on error to `select` path but the app
loads the select's code on `apply` path only. To solve that problem
we share this code to both paths.

Closes https://gitlab.com/gitlab-org/gitlab/issues/33740

ee/app/assets/javascripts/pages/trials/apply/index.js

+import 'ee/pages/trials/namespace_select';

ee/app/assets/javascripts/pages/trials/namespace_select.js

+import $ from 'jquery';
+
+document.addEventListener('DOMContentLoaded', () => {
+  const namespaceId = $('#namespace_id');
+  const newGroupName = $('#group_name');
+
+  namespaceId.on('change', () => {
+    const enableNewGroupName = namespaceId.val() === '0';
+
+    newGroupName
+      .toggleClass('hidden', !enableNewGroupName)
+      .find('input')
+      .prop('required', enableNewGroupName);
+  });
+
+  namespaceId.trigger('change');
+});

ee/app/assets/javascripts/pages/trials/select/index.js

-import $ from 'jquery';
-
-document.addEventListener('DOMContentLoaded', () => {
-  const namespaceId = $('#namespace_id');
-  const newGroupName = $('#group_name');
-
-  namespaceId.on('change', () => {
-    const enableNewGroupName = namespaceId.val() === '0';
-
-    newGroupName
-      .toggleClass('hidden', !enableNewGroupName)
-      .find('input')
-      .prop('required', enableNewGroupName);
-  });
-
-  namespaceId.trigger('change');
-});
+import 'ee/pages/trials/namespace_select';

ee/spec/features/trials/select_namespace_spec.rb

@@ -84,13 +84,14 @@ describe 'Trial Select Namespace', :js do
           click_button 'Start your free trial'
 
           message = page.find('#new_group_name').native.attribute('validationMessage')
+
           expect(message).to eq('Please fill out this field.')
           expect(current_path).to eq(select_trials_path)
         end
       end
     end
 
-    context 'selects an existing user' do
+    context 'selects an existing group' do
       before do
         visit select_trials_path
         wait_for_all_requests
@@ -98,20 +99,42 @@ describe 'Trial Select Namespace', :js do
         select2 user.namespace.id, from: '#namespace_id'
       end
 
-      it 'does not show the new group name input' do
-        expect(page).not_to have_field('New Group Name')
-      end
+      context 'without trial plan' do
+        it 'does not show the new group name input' do
+          expect(page).not_to have_field('New Group Name')
+        end
+
+        it 'applies trial and redirects to dashboard' do
+          expect_any_instance_of(GitlabSubscriptions::ApplyTrialService).to receive(:execute) do
+            { success: true }
+          end
+
+          click_button 'Start your free trial'
+
+          wait_for_requests
 
-      it 'applies trial and redirects to dashboard' do
-        expect_any_instance_of(GitlabSubscriptions::ApplyTrialService).to receive(:execute) do
-          { success: true }
+          expect(current_path).to eq("/#{user.namespace.path}")
         end
+      end
 
-        click_button 'Start your free trial'
+      context 'with trial plan' do
+        let!(:error_message) { 'Validation failed: Gl namespace can have only one trial' }
 
-        wait_for_requests
+        it 'shows validation error' do
+          expect_any_instance_of(GitlabSubscriptions::ApplyTrialService).to receive(:execute) do
+            { success: false, errors: error_message }
+          end
+
+          click_button 'Start your free trial'
 
-        expect(current_path).to eq("/#{user.namespace.path}")
+          expect(find('.flash-text')).to have_text(error_message)
+          expect(current_path).to eq(apply_trials_path)
+
+          # new group name should be functional
+          select2 '0', from: '#namespace_id'
+
+          expect(page).to have_field('New Group Name')
+        end
       end
     end
   end