Retry Refine Digest

[ksss]

This PR is a retry of https://github.com/ruby/rbs/pull/1779 .

I have stopped returning bot. Then I changed testing method to raap. The Digest classes require a lot of changes when modifying RBS. If the behavior is mechanical, using raap to handle these changes can reduce management costs compared to manually updating test code. This allows us to focus on changing the signatures.