fix(scanner): fence timed out scan cycles (#6352)

* fix(scanner): fence timed out scan cycles

* fix(scanner): cancel scan workers with cycle scope

* fix(scanner): reject persisted timer overflow

* fix(scanner): reject terminal leadership epochs

* fix(scanner): reject trailing cycle state bytes

crates/scanner/src/scanner_io.rs

@@ -48,6 +48,7 @@ use time::OffsetDateTime;
 use tokio::sync::{Mutex, Notify, Semaphore, mpsc};
 use tokio::time::Duration;
 use tokio_util::sync::CancellationToken;
+use tokio_util::task::AbortOnDropHandle;
 use tracing::{debug, error, warn};
 
 use crate::ScannerObjectInfo as ObjectInfo;