// RestClient client = builder.build();  // no basic auth
// client.get().uri(baseUrl + "/api/hello").retrieve().body(String.class);

Thrown: org.springframework.web.client.HttpClientErrorException$Unauthorized
Status: 401 UNAUTHORIZED

# retrieve() throws HttpClientErrorException on 4xx by default -- same default as
# RestTemplate, unlike RestClient's own exchange() method, which disables that default.