fix: skip dunder prefixed fields while adding child docs to locals

these include fields like __print_formats, __workflow_docs which do not actually contain child docs for the doctype.

caused issue #18581 where the local print format doc was being unlinked from cur_frm while sycing as it also synced __print_formats.

frappe/public/js/frappe/model/sync.js

@@ -89,6 +89,7 @@ Object.assign(frappe.model, {
 		// add child docs to locals
 		if (!is_table) {
 			for (var i in doc) {
+				if (i.startsWith("__")) continue;
 				var value = doc[i];
 
 				if ($.isArray(value)) {