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/componentStructureHydrate.ts

@@ -78,10 +78,18 @@ export const hydrateCustomFields = (fields: any[]): any[] => {
     const customFieldId = typeof field?.customFieldId === 'string' ? field.customFieldId : undefined
     const orderIndex = typeof field?.orderIndex === 'number' ? field.orderIndex : index
 
+    const machineContextOnly =
+      typeof field?.machineContextOnly === 'boolean'
+        ? field.machineContextOnly
+        : typeof valueObject?.machineContextOnly === 'boolean'
+          ? valueObject.machineContextOnly
+          : false
+
     return {
       name,
       type,
       required,
+      machineContextOnly,
       options,
       optionsText,
       defaultValue,
@@ -153,6 +161,7 @@ export const mapComponentCustomFields = (fields: any[]) => {
       name: typeof field?.name === 'string' ? field.name : '',
       type: field?.type ?? 'text',
       required: !!field?.required,
+      machineContextOnly: !!field?.machineContextOnly,
       options: Array.isArray(field?.options) ? field.options : [],
       optionsText: typeof field?.optionsText === 'string' ? field.optionsText : '',
       defaultValue,