feat(validation): disallow SetNull referential action on required referenced fields

[jkomyno]

This PR validates against required fields referenced by a relation with at least one SetNull referential action, regardless of whether it appears in onUpdate or onDelete.

E.g., with the following schema,

model SomeUser {
  id      Int      @id
  ref     Int
  profile Profile?
  @@unique([id, ref])
}

model Profile {
  id       Int       @id
  user     SomeUser? @relation(fields: [user_id, user_ref], references: [id, ref], onDelete: SetNull)
  user_id  Int?
  user_ref Int
  @@unique([user_id, user_ref])
}

this PR introduces the following validation error:

The onDelete referential action of a relation must not be set to SetNull when a referenced field is required.
Either choose another referential action, or make the referenced fields optional.

Note: the only SQL database we support which would accept the DDL declaration equivalent to the Prisma schema above is Postgres, although it would still fail at runtime when the SET NULL referential action is triggered.
This PR, however, introduces this new validation regardless of the used database connector. See internal doc for more details. It should be noted that Tom has expressed some concerns on validating against something that Postgres actually supports (although we argue that Postgres' support in this case is a bug).

Closes prisma/prisma#14673.

[Jolg42]

Looks good to me though, let's confirm with someone with more knowledge in the engines.

[pimeys]

A few nitpicks, also somebody from the @prisma/client-team-rust should ack the removed tests.

[Weakky]

Removed tests make sense to me considering the new validation rule 👍

[Jolg42]

Note: let's monitor introspection-ci later

[Jolg42]

See introspection-ci results https://prisma-company.slack.com/archives/C01778YKS00/p1666686897741579