DEV: Convert admin user fields to FormKit (#29070)

This change replaces the admin form for adding and editing custom user fields with a new FormKit implementation.

spec/system/page_objects/components/form_kit.rb

@@ -67,6 +67,16 @@ module PageObjects
         expect(self.value).to eq(expected_value)
       end
 
+      def has_errors?(*messages)
+        within component do
+          messages.all? { |m| find(".form-kit__errors", text: m) }
+        end
+      end
+
+      def has_no_errors?
+        !has_css?(".form-kit__errors")
+      end
+
       def control_type
         component["data-control-type"]
       end

spec/system/page_objects/pages/admin_user_fields.rb

@@ -8,6 +8,10 @@ module PageObjects
         self
       end
 
+      def form
+        PageObjects::Components::FormKit.new(".user-field .form-kit")
+      end
+
       def choose_requirement(requirement)
         form = page.find(".user-field")