Verifies that a data-carrying enum has its variant fields registered in the app
schema with globally-assigned field indices (indices are unique across all variants).
End-to-end CRUD test for a data-carrying enum (all variants have fields).
Creates records with different variants, reads them back, and verifies roundtrip.
Tests that jiff::Timestamp fields inside data-carrying enum variants round-trip correctly.
Also covers a mixed enum (one unit variant, one data variant) to verify null handling.
Tests that UUID fields inside data-carrying enum variants round-trip correctly.
UUID is a non-trivial primitive that requires type casting on some databases.
Roundtrip test for an enum embedded inside a variant field of another enum (enum-in-enum).
The inner enum is unit-only; the outer has one data variant and one unit variant.
Verifies that a mixed enum (some unit variants, some data variants) registers
correctly: unit variants have empty fields, data variants have their fields
with indices assigned starting from 0 and continuing globally across variants.