Is there a way to write tests to prove that only the api configuration is exposed to consuming projects, and that the implementation configuration jars are not leaked?

Why do you want to test that Gradle works as expected? Or what is your intention?

I am having a hard time trying to find a way to test that I only declare what's needed for the api configuration so only the intended classes are exposed to consumers and no more. I thought I'd write a test that only had "api" and not "implementation", but I am not finding a way to do that, and the diagram says that too. To have a test with just the the api classes, it's not possible (have to get implementation too).

Just use https://github.com/autonomousapps/dependency-analysis-android-gradle-plugin/ then. It tells you if you have dependencies in api that you should have in implementation and many other things.

Wow, that plugin really is something. Thank you @Vampire! ❤️