Function validate_backup_deletion_policy 

pub fn validate_backup_deletion_policy(
    origin: Origin,
    policy: Option<DeletionPolicy>,
) -> ValidationResult

A Backup’s deletionPolicy is legal for its origin (ADR §4.5).

origin: discovered forces Retain: None (defaults to Retain) and an explicit Retain pass; Delete/Orphan are rejected. Other origins accept any policy.