Can't mock Beans in Micronaut Serverless Function tests using Mockito

Question

I am super new to Micronaut and I am trying a couple of things out. My end goal is to be able to deploy a Micronaut Serverless Function, built as a Native Image on AWS Lambda.

I have gone through the documentation, however, I am struggling to mock Beans in my tests. Currently, I am only trying this out with a Sample Program where I want to mock out a method call from my service. I am getting a null response for my mocked method when using Mockito in tests.

package com.example;
import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent;
import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEvent;
import io.micronaut.context.ApplicationContext;
import io.micronaut.core.annotation.Introspected;
import io.micronaut.function.aws.MicronautRequestHandler;
import jakarta.inject.Inject;

@Introspected
public class BookRequestHandler extends MicronautRequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    @Inject
    private BookService bookService;

    public BookRequestHandler() {

    }

    public BookRequestHandler(ApplicationContext applicationContext) {
        super(applicationContext);
    }

    @Override
    public APIGatewayProxyResponseEvent execute(APIGatewayProxyRequestEvent request) {
        return new APIGatewayProxyResponseEvent()
                .withStatusCode(200)
                .withBody(bookService.sayHello());
    }
}

public interface BookService {
    String sayHello();
}
@Singleton
public class BookServiceImpl implements BookService {
    public String sayHello() {
        return "HELLO";
    }
}

@MicronautLambdaTest
public class BookRequestHandlerTest {

    @Inject
    private BookService bookService;

    @Inject
    private ApplicationContext applicationContext;

    @Test
    public void testHandler() {
        BookRequestHandler bookRequestHandler = new BookRequestHandler(applicationContext);
        when(bookService.sayHello()).thenReturn("MOCKED RESPONSE");
        APIGatewayProxyResponseEvent responseEvent =
                bookRequestHandler.execute(new APIGatewayProxyRequestEvent());
        assertEquals(responseEvent.getStatusCode(), 200);
        assertEquals(responseEvent.getBody(), "MOCKED RESPONSE");
    }

    @MockBean(BookServiceImpl.class)
    BookService bookService() {
        return mock(BookService.class);
    }
}

I would really appreciate it if someone could point me towards the best practice for Mocking Beans in Micronaut when developing Serverless Functions because currently, I am not able to stub my methods.

Moreover, is using @MicronautLambdaTest recommended while testing Serverless Functions? There seems to be some discrepancy between the Sample Code generated from launch and the AWS Documentation.

Any help is much appreciated :D