fix(custom-fields) : preserve machineContextOnly through sanitizeCustomFields

The sanitize layer was reconstructing the field object without machineContextOnly,
causing it to always be false in the save payload regardless of checkbox state.

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

frontend/app/shared/model/componentStructureSanitize.ts

@@ -85,7 +85,10 @@ export const sanitizeCustomFields = (fields: any[]): ComponentModelCustomField[]
         }
       }
 
-      const result: ComponentModelCustomField = { name, type, required }
+      const machineContextOnly =
+        typeof valueObject?.machineContextOnly === 'boolean' ? valueObject.machineContextOnly : !!field?.machineContextOnly
+
+      const result: ComponentModelCustomField = { name, type, required, machineContextOnly }
       if (options) {
         result.options = options
       }