Methods defined on `PrivateRelation` should return `T.self_type`

I wrote a compiler for Kaminari that returns the following:

sig { T.all(PrivateRelation, Kaminari::PageScopeMethods, Kaminari::ActiveRecordRelationMethods) }
def page(num = nil); end

This type is accurate, since .page uses .extending to add methods to the relation.

This works:

User.page(5).total_count

This doesn't work:

User.page(5).all.total_count
                 ^^^^^^^^^^^ Method `total_count` does not exist on `User::PrivateRelation`

That's because the return type of for .all is PrivateRelation. We could fix this problem by using T.self_type instead, which I believe more accurately captures the way ActiveRecord works.

The way that relevant compilers are currently implemented makes this a little tricky, since the methods that are being added to the PrivateRelation are also added to the model. The methods defined on the model should be PrivateRelation.

Start by examining the relevant compilers that add methods to PrivateRelation and the model, then trace how the return type for .all is generated. Verify the behavior against the Kaminari example, including chaining .all and preserving the extended relation methods. Done means the generated type reflects the relation's self type without losing methods added to the model.