fix(custom-fields) : pass machineContextOnly through structure serialize/hydrate

- componentStructure: include machineContextOnly in save payload value object
- componentStructureHydrate: read machineContextOnly in hydrate and mapComponentCustomFields
- pieceProductStructure: include machineContextOnly in sanitize and hydrate

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app/shared/model/componentStructure.ts

@@ -141,6 +141,7 @@ export const normalizeStructureForSave = (input: any): any => {
     const value: Record<string, any> = {
       type: field.type,
       required: !!field.required,
+      machineContextOnly: !!field.machineContextOnly,
     }
     if (field.options && field.options.length) {
       value.options = field.options