fix(ecstore): mint bucket-target ARNs in the madmin arn:minio partition (#6128)

* test(ecstore): pin madmin-compatible ARN partition contract

Red-light evidence for backlog#1675 P1-7: madmin-go's ParseARN
hard-rejects any ARN that does not start with 'arn:minio:', while RustFS
generates and only accepts 'arn:rustfs:'. mc/madmin tooling therefore
cannot decode RustFS remote-target listings, and MinIO-era replication
configs are rejected as StaleTarget when re-registered. The new tests
pin the target contract (generate arn:minio:, parse both partitions,
reject unknown partitions) and fail against the current single-partition
gate.

* fix(ecstore): mint bucket-target ARNs in the madmin arn:minio partition

madmin-go's ParseARN hard-rejects any partition other than 'arn:minio:',
so native mc/madmin tooling could not decode RustFS remote-target
listings, and re-registering a MinIO-era replication config failed its
StaleTarget check against freshly minted arn:rustfs: targets
(backlog#1675 P1-7, route A).

- ARN Display now emits 'arn:minio:'; FromStr accepts a {minio, rustfs}
  partition whitelist (the legacy partition stays readable forever for
  persisted bucket-targets.json / replication configs). The whitelist is
  the only structural gate — BucketTargetType::from_str never fails —
  so it deliberately rejects foreign partitions such as arn:aws:.
- No data migration: every runtime match between targets, rules and
  stats keys is full-string equality, so existing arn:rustfs: targets
  keep matching their persisted rules; site replication already
  preserves MinIO-era ARNs on reconcile (pinned by existing tests).
- Rolling upgrade note: upgrade all cluster nodes before creating new
  remote targets — a not-yet-upgraded node rejects remove-remote-target
  for a freshly minted arn:minio: ARN with BucketRemoteArnInvalid.
- Out of scope: notification/SQS ARNs (crates/targets) keep the
  arn:rustfs:sqs: partition; they have their own compatibility story.

rustfs/src/admin/handlers/site_replication.rs

@@ -15305,7 +15305,10 @@ mod tests {
         assert!(!target.secure);
         assert_eq!(target.target_bucket, "photos");
         assert_eq!(target.deployment_id, "remote");
-        assert_eq!(target.arn, "arn:rustfs:replication::remote:photos");
+        // Freshly minted ARNs use the `minio` partition so madmin-go tooling
+        // can parse them; legacy `arn:rustfs:` targets are preserved as-is
+        // (see the MinIO-era preservation test below).
+        assert_eq!(target.arn, "arn:minio:replication::remote:photos");
         assert_eq!(target.region, "us-east-1");
         let credentials = target
             .credentials