[400] AuthenticationFailureHandler (인증 실패 핸들러: 로그인 실패 핸드러) @Slf4j@RequiredArgsConstructorpublic class LoginFailHandler implements AuthenticationFailureHandler { private final ObjectMapper objectMapper; @Override public void onAuthenticationFailure(HttpServletRequest request, HttpServletResponse response, AuthenticationException exception) throws IOException, ServletException { lo..