docs: HttpServerExchange startResponse() throws IllegalStateException {
          int oldVal = state;
          if (allAreSet(oldVal, FLAG_RESPONSE_SENT)) {
              throw UndertowMessages.MESSAGES.responseAlreadyStarted();
          }
          this.state = oldVal | FLAG_RESPONSE_SENT;
  
          log.tracef("Starting to write response for %s", this);
          return this;
      }