<fix>[errorcode]: revert sendReplyResponse to use JSONObjectUtil serialization

CloudBusGson.httpGson does not disableHtmlEscaping(), causing single
quotes in error details to be escaped as \u0027. While valid JSON,
the SDK deserializes these literally, breaking string assertions in
tests like BatchAddBareMetal2ChassisCase that check error.details
with contains("'reboot' must be 'No'").

Restore JSONObjectUtil.toJsonString() for the sendReplyResponse error
path while keeping the centralized localization logic.

Co-Authored-By: ye.zou <ye.zou@zstack.io>

Author: AlanJager

rest/src/main/java/org/zstack/rest/RestServer.java

@@ -1439,8 +1439,13 @@ private void sendReplyResponse(MessageReply reply, Api api, HttpServletResponse
         ApiResponse response = new ApiResponse();
 
         if (!reply.isSuccess()) {
+            String locale = resolveLocale();
+            i18nService.localizeErrorCode(reply.getError(), locale);
             response.setError(reply.getError());
-            sendResponse(HttpStatus.SERVICE_UNAVAILABLE.value(), response, rsp);
+            // use JSONObjectUtil (which disables HTML escaping) to keep the same
+            // serialization behavior as before; CloudBusGson.httpGson escapes '\'' to
+            // '\u0027' which breaks SDK-side string assertions (ZSTAC-71075 etc.)
+            sendResponse(HttpStatus.SERVICE_UNAVAILABLE.value(), JSONObjectUtil.toJsonString(response), rsp);
             return;
         }