Set iids on before_save callbacks

What does this MR do?

Related to #292225 (closed)

Prevent edge cases described in the related issue by moving the generation of iid values to before_create and before_update rather than before_validation.

Since iid generation only happens once the model is valid according to application logic, a failure to save at that point should result in an exception/transaction rollback. To prevent the application from catch/retry such an error, use an after_rollback callback to unset the iid value on the model if it was previously set in the ensure_#{scope}_iid! method.

In cases where the iid value was already persisted on the model, or where the ensure_iid! logic was skipped, don't clear the attribute on the model.

Does this MR meet the acceptance criteria?

Conformity

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