fix: use frm.set_value for updating fields (#22860)

Just updating frm.doc object doesn't update the underlying model. In Frappe `frm.doc` is NOT the primary representation or source of truth for better or worse.

frappe/public/js/form_builder/store.js

@@ -201,7 +201,7 @@ export const useStore = defineStore("form-builder-store", () => {
 			let fields = get_updated_fields();
 			let has_error = validate_fields(fields, doc.value.istable);
 			if (has_error) return has_error;
-			doc.value.fields = fields;
+			frm.value.set_value("fields", fields);
 			return fields;
 		} catch (e) {
 			console.error(e);