fix[Op#57579]: stick to select_custom_fields perm check

Removes the admi only guard as it's possible project admins (who are'nt admins) can modify
custom field activations at project level. Hence restricting to just admins does not make sense

app/contracts/custom_fields/custom_field_projects/base_contract.rb

@@ -29,8 +29,6 @@
 module CustomFields
   module CustomFieldProjects
     class BaseContract < ::ModelContract
-      include RequiresAdminGuard
-
       attribute :project_id
       attribute :custom_field_id
 
@@ -47,7 +45,7 @@ def not_for_all
         # Only mappings of custom fields which are not enabled for all projects can be manipulated by the user
         return if model.custom_field.nil? || !model.custom_field.is_for_all?
 
-        errors.add :custom_field_id, :cannot_delete_mapping
+        errors.add :custom_field_id, :is_for_all_cannot_modify
       end
     end
   end

config/locales/en.yml

@@ -884,6 +884,7 @@ en:
         blank: "can't be blank."
         blank_nested: "needs to have the property '%{property}' set."
         cannot_delete_mapping: "is required. Cannot be deleted."
+        is_for_all_cannot_modify: "is for all. Cannot be modified."
         cant_link_a_work_package_with_a_descendant: "A work package cannot be linked to one of its subtasks."
         circular_dependency: "This relation would create a circular dependency."
         confirmation: "doesn't match %{attribute}."

spec/contracts/custom_fields/custom_field_projects/base_contract_spec.rb

@@ -42,13 +42,25 @@
     let(:custom_field) { build_stubbed(:custom_field, is_for_all: true) }
     let(:custom_field_project) { build_stubbed(:custom_fields_project, custom_field:) }
 
-    it_behaves_like "contract is invalid"
+    it_behaves_like "contract is invalid", custom_field_id: :is_for_all_cannot_modify
   end
 
-  context "with non-admin user" do
+  context "with authorised user" do
     let(:user) { build_stubbed(:user) }
 
-    it_behaves_like "contract is invalid", base: %i[error_unauthorized error_unauthorized]
+    before do
+      mock_permissions_for(user) do |mock|
+        mock.allow_in_project(:select_custom_fields, project: custom_field_project.project)
+      end
+    end
+
+    it_behaves_like "contract is valid"
+  end
+
+  context "with unauthorised user" do
+    let(:user) { build_stubbed(:user) }
+
+    it_behaves_like "contract is invalid", base: :error_unauthorized
   end
 
   include_examples "contract reuses the model errors"