Allow beans created in MockRestServiceServerAutoConfiguration to be replaced by user-provided alternatives

[HuitaePark]

Related Issue

Closes gh-46853

Goal

Allow overriding the MockServerRestClientCustomizer bean created by MockRestServiceServerAutoConfiguration.

This is particularly useful for scenarios in 'RestClientTest' where customization of the mock server behavior is required, such as enabling content buffering via setBufferContent(true), which was previously overridden by the default auto-configuration.

Background

Currently, MockRestServiceServerAutoConfiguration unconditionally defines a MockServerRestClientCustomizer bean:

@Bean
MockServerRestClientCustomizer mockServerRestClientCustomizer() {
    return new MockServerRestClientCustomizer();
}

Because of this unconditional registration, if a user attempts to provide their own MockServerRestClientCustomizer bean to customize the MockRestServiceServer, the auto-configured bean may take precedence or cause conflicts. This prevents users from applying necessary configurations like enabling the buffering of response bodies.

Approach

Apply conditional registration:

Added @ConditionalOnMissingBean to the mockServerRestClientCustomizer bean definition in MockRestServiceServerAutoConfiguration.

This ensures that the auto-configuration backs off if a user has already defined a MockServerRestClientCustomizer bean in the application context.

Verification

I introduced a new test class MockRestServiceServerAutoConfigurationTests using ApplicationContextRunner to verify the fix.

The tests cover two scenarios:

1. Default Behavior: Verifies that MockServerRestClientCustomizer is registered by the auto-configuration when no user bean is present.

2. Override Behavior: Verifies that the auto-configuration backs off when a user-provided MockServerRestClientCustomizer is present, ensuring the user's bean is used.

[wilkinsona]

Thanks very much for the PR, @HuitaePark. I've left some comments for your consideration.

[HuitaePark]

@wilkinsona Thanks for the review. I've applied the changes.

[wilkinsona]

Thanks very much, @HuitaePark.