chore(obs): rename ReplicationStats to ReplicationMetricsSnapshot (#6344)

* chore(obs): ReplicationStats -> ReplicationMetricsSnapshot, BucketReplicationStats -> BucketReplicationMetricsSnapshot

Rename in-obs-crate ReplicationStats and BucketReplicationStats to
ReplicationMetricsSnapshot and BucketReplicationMetricsSnapshot respectively.
No serde impact (these types are Prometheus metric collectors, not serialized).
No external consumers found outside the obs crate.

* cleanup: remove #[serial] annotations from e2e_test, scanner, lifecycle, and object-capacity crates

Remove no-op #[serial] attributes (nextest ignores serial_test) and the
serial_test dependency from four crates. All tests already use temp_env
for env-var isolation, making #[serial] purely redundant.

Crates cleaned:
- e2e_test (37 annotations, 9 imports, removed serial_test dep)
- rustfs-scanner (115 annotations across 7 files, removed serial_test dep)
- rustfs-lifecycle (46 annotations, removed serial_test dep)
- rustfs-object-capacity (38 annotations, removed serial_test dep)

Also converted scanner/tests/lifecycle_integration_test.rs
with_forced_immediate_enqueue_timeout helper from unsafe raw
env::set_var/remove_var to temp_env::async_with_vars for proper
isolation, and added async_closure feature to scanner's temp-env dep.

* fix(lifecycle): restore #[serial] on 2 tests that read env vars without temp_env

eval_inner_expires_latest_object_after_days_due and
eval_inner_does_not_panic_on_many_equal_due_events call eval_inner()
which reads ENV_ILM_PROCESS_TIME via std::env::var(). Without #[serial]
they race with other tests that set these vars via temp_env.

* style: cargo fmt

crates/object-capacity/src/scan.rs

@@ -1069,7 +1069,6 @@ mod tests {
     #[cfg(unix)]
     use rustfs_config::ENV_CAPACITY_FOLLOW_SYMLINKS;
     use rustfs_config::{ENV_CAPACITY_MAX_FILES_THRESHOLD, ENV_CAPACITY_SAMPLE_RATE};
-    use serial_test::serial;
 
     /// Reference implementation using unbounded `u128` arithmetic, clamped to
     /// `u64::MAX`, used as the source of truth for the sampling extrapolation.
@@ -1274,7 +1273,6 @@ mod tests {
     }
 
     #[tokio::test]
-    #[serial]
     async fn test_get_dir_size_async_nonexistent_directory() {
         let result = get_dir_size_async(Path::new("/nonexistent/path")).await;
         assert!(result.is_err());
@@ -1648,7 +1646,6 @@ mod tests {
     }
 
     #[tokio::test]
-    #[serial]
     async fn test_metadata_incomplete_aggregate_does_not_replace_disk_cache() {
         use std::fs::File;
         use std::io::Write;
@@ -1783,7 +1780,6 @@ mod tests {
 
     #[cfg(unix)]
     #[tokio::test]
-    #[serial]
     async fn test_get_dir_size_async_ignores_symlink_targets_when_follow_disabled() {
         use std::fs::File;
         use std::io::Write;
@@ -1809,7 +1805,6 @@ mod tests {
 
     #[cfg(unix)]
     #[tokio::test]
-    #[serial]
     async fn test_get_dir_size_async_counts_symlink_targets_when_follow_enabled() {
         use std::fs::File;
         use std::io::Write;