@Override
    public Mono<User> getUser(String username) {
        return client.get(User.class, username)
            .onErrorMap(ExtensionNotFoundException.class, e -> new UserNotFoundException(username));
    }
@Test
    void shouldGetUserIfUserFoundInExtension() {
        User fakeUser = new User();
        when(client.get(User.class, "faker")).thenReturn(Mono.just(fakeUser));

        StepVerifier.create(userService.getUser("faker"))
            .assertNext(user -> assertEquals(fakeUser, user))
            .verifyComplete();

        verify(client, times(1)).get(eq(User.class), eq("faker"));
    }