refactor(replication): name the resync state error and keep io failures typed (#6628)

Backlog#1845 step 7. The replication crate's hand-rolled, crate-generic Error type actually describes one thing: failures of the persisted resync/MRF state files. Rename it to ResyncStateError so the name says so, and stop collapsing io::Error into Other(String): a new Io(std::io::Error) variant keeps the kind and source chain, Display renders identically, and the ecstore boundary maps it to StorageError::Io so the kind survives into store-layer classification instead of degrading into a stringified other().

No thiserror introduced - the crate keeps its zero-internal-deps posture and hand-written impls.

Ref rustfs/backlog#1845

crates/replication/src/lib.rs

@@ -78,7 +78,7 @@ pub use queue::{
     replication_heal_queue_action, worker_queue_for_replication_type,
 };
 pub use resync::{
-    BucketReplicationResyncStatus, Error, RESYNC_FILE_MAX_BYTES, Result, ResyncOpts, ResyncStatusType,
+    BucketReplicationResyncStatus, RESYNC_FILE_MAX_BYTES, Result, ResyncOpts, ResyncStateError, ResyncStatusType,
     TargetReplicationResyncStatus, decode_resync_file, encode_resync_file, is_version_id_mismatch, resync_state_accepts_update,
     resync_status_duration, sanitize_resync_error_detail, should_auto_resume_resync, should_count_head_proxy_failure,
 };