Prevent `prisma:engine:response_json_serialization` span from being
orphan when using interactive transactions.

This check made sense in the past in principle when there were no child
spans outside of the iTX runner, and the `prisma:engine` span didn't
cover the whole `Engine::query` method duration, but now it makes more
sense to always create it.

Fixes: prisma/prisma#21402

…gardless-of-itx