fix(custom-fields) : include machineContextOnly in ModelType.serializeCustomFields

The GET response for ModelType structure was missing machineContextOnly,
so on page reload the flag was always read as false by the frontend.

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

src/Entity/ModelType.php

@@ -428,6 +428,7 @@ class ModelType
                 'options'            => $cf->getOptions(),
                 'defaultValue'       => $cf->getDefaultValue(),
                 'orderIndex'         => $cf->getOrderIndex(),
+                'machineContextOnly' => $cf->isMachineContextOnly(),
             ];
         }