Make workflow custom fields editable (#12063)

Fixes https://github.com/twentyhq/twenty/issues/11989

<img width="1267" alt="Capture d’écran 2025-05-15 à 14 05 15"
src="https://github.com/user-attachments/assets/fbb22f52-2c76-424f-8b8c-fb030fef9fa8"
/>

`isCustom` was not properly set everywhere because it was not mandatory
in `isFieldValueReadOnly`. Removing the default value and adding it to
missing places

packages/twenty-front/src/modules/activities/components/ActivityRichTextEditor.tsx

@@ -70,6 +70,7 @@ export const ActivityRichTextEditor = ({
   const isReadOnly = isFieldValueReadOnly({
     objectNameSingular: activityObjectNameSingular,
     isRecordReadOnly,
+    isCustom: objectMetadataItemActivity.isCustom,
   });
 
   const { deleteManyRecords: deleteAttachments } = useDeleteManyRecords({