Verify overloads of `==`, `hash` and `*_coder` methods

Going to rails 8.1, I saw that we were missing some overrides, which made some tests flaky due to rails' deduplicable logic. But mostly, which was wrong!

Since we want to publish 8.1 quickly, and the bug is already here anyway. I'll mark the code with some `TODO` referencing this issue. We'll need to both address theses TODOs and check if there are other places where deduplication is failing.