Function cleanup_on_error 

async fn cleanup_on_error<E, C>(
    partial_path: Option<&PathBuf>,
    request_id: &str,
    registry: &TransferRegistryHandle,
    lock_registry: &LockRegistry,
    lock_id: &LockId,
    channel: &E,
    error: AppError,
    log: &NotificationLogHandle,
    os_notifier: &OsNotifier<C>,
)
where
    E: EventEmitter,
    C: OsNotifyChannel,

Error path: delete .partial (if any), emit Failed, release lock.