Improve support for `@Nested` and `@NestedTestConfiguration` in various test framework contracts

Overview

It's unclear to me exactly where support for @Nested and @NestedTestConfiguration fits into the test framework's various contracts. Clarifying that would, I hope, make it easier to understand which APIs and utility methods support @Nested and which require implementers of those contracts to roll their own support. Hopefully the situations where implementers need to roll their own support can then be reduced and, ideally, eliminated entirely.

One specific example is AnnotationConfigContextLoaderUtils. I'm rather confused about the purpose and general utility of AnnotationConfigContextLoaderUtils if it doesn't support @Nested and @NestedTestConfiguration semantics.

Perhaps this would be better handled as two issues? One to make some additions to the javadoc to describe the current situation and steer implementers in the right direction and a second to reduce the cases where implementers need to do anything at all.

Related Issues

• #31456

Start by reading AnnotationConfigContextLoaderUtils and the related issue #31456, then map how @Nested and @NestedTestConfiguration are handled across the test framework contracts. Determine whether the work should clarify the current behavior in Javadoc, reduce the need for implementers to add support, or both; done means the relevant contracts and utility behavior are clearly documented and supported consistently.