@Transactional(readOnly = true)
    public void validateCouponExisted(final Long couponId) {
        if (!couponRepository.isExistedById(couponId)) {
            throw new CouponNotFoundException();
        }
    }
@Test
    void 쿠폰이_존재하지_않으면_예외를_발생시킨다() {
        // when & then
        assertThatThrownBy(() -> couponService.validateCouponExisted(-1L))
                .isInstanceOf(CouponNotFoundException.class);
    }