Prior to this commit, tests using the `@WebMvcTest` annotation would not
include the `HttpEncodingAutoConfiguration`. This means that, even if
configured, the encoding filter would not be configured in MVC tests,
resulting in an inconsistency with `@SpringBootTest` tests.

This commit ensures that the `HttpEncodingAutoConfiguration` is included
when `@WebMvcTest` is used.

Fixes gh-23749