How to use MockMvc and MockRestServiceServer in Java @SpringBootTest

I am attempting to write a @SpringBootTest which will both issue REST requests and mock responses. I want to do this because my spring application receives REST requests and in response it fetches information from another source using REST. I would like to trigger my application by initiating a REST GET (MockMvc), but would also like to mock the other source's response (MockRestServiceServer).

Here is a sanitized version of the test, with my application removed:

import lombok.extern.slf4j.Slf4j;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.context.TestConfiguration;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Primary;
import org.springframework.http.MediaType;
import org.springframework.test.context.junit.jupiter.SpringExtension;
import org.springframework.test.web.client.MockRestServiceServer;
import org.springframework.test.web.servlet.MockMvc;
import org.springframework.web.client.RestTemplate;

import static org.hamcrest.Matchers.containsString;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
import static org.springframework.test.web.client.match.MockRestRequestMatchers.requestTo;
import static org.springframework.test.web.client.response.MockRestResponseCreators.withSuccess;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@Slf4j
@ExtendWith(SpringExtension.class)
@SpringBootTest
@AutoConfigureMockMvc
class RestTest {
    private static final String URI = "/my-test-uri";
    private static final String RESPONSE = "my-response";
    @Autowired
    private MockMvc mockMvc;

    @Test
    void testRest() throws Exception {
        log.info("Initiating request for GET {}", URI);
        mockMvc.perform(get(URI))
                .andExpect(status().isOk())
                .andExpect(content().string(containsString(RESPONSE)));
    }

    @TestConfiguration
    static class TestConfig {

        @Primary
        @Bean
        public RestTemplateBuilder restTemplateBuilder() {
            RestTemplate restTemplate = new RestTemplate();
            MockRestServiceServer server = MockRestServiceServer.createServer(restTemplate);
            server.expect(requestTo(URI))
                    .andRespond(withSuccess(RESPONSE, MediaType.APPLICATION_JSON));
            log.info("Mocking response to GET {}: {}", URI, RESPONSE);

            RestTemplateBuilder mockBuilder = mock(RestTemplateBuilder.class);
            when(mockBuilder.build()).thenReturn(restTemplate);
            return mockBuilder;
        }
    }
}

The test fails because it does not receive the response defined in TestConfig.restTemplateBuilder. I am providing both the REST requestor (testRest() method) and the REST responder (TestConfig.restTemplateBuilder). What am I doing wrong?

Here's the failure:

Status expected:<200> but was:<404>
Expected :200
Actual   :404
<Click to see difference>

java.lang.AssertionError: Status expected:<200> but was:<404>
	at org.springframework.test.util.AssertionErrors.fail(AssertionErrors.java:59)
	at org.springframework.test.util.AssertionErrors.assertEquals(AssertionErrors.java:122)
	at org.springframework.test.web.servlet.result.StatusResultMatchers.lambda$matcher$9(StatusResultMatchers.java:627)
	at org.springframework.test.web.servlet.MockMvc$1.andExpect(MockMvc.java:196)
	at com.mycompany.RestTest.testRest(RestTest.java:54)
...
	at java.base/java.lang.Thread.run(Thread.java:830)