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/scanner/src/sleeper.rs

@@ -258,7 +258,6 @@ impl SleepTimer {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use serial_test::serial;
     use temp_env::{with_var, with_var_unset};
 
     struct ScannerDefaultSpeedGuard;
@@ -326,7 +325,6 @@ mod tests {
     }
 
     #[test]
-    #[serial]
     fn test_refresh_from_env_applies_speed_and_idle_mode_for_next_cycle() {
         let prev_mode = SCANNER_IDLE_MODE.load(Ordering::Relaxed);
         SCANNER_IDLE_MODE.store(true, Ordering::Relaxed);
@@ -346,7 +344,6 @@ mod tests {
     }
 
     #[test]
-    #[serial]
     fn test_refresh_from_env_uses_default_speed_override_when_speed_unset() {
         let _guard = ScannerDefaultSpeedGuard::set(ScannerSpeed::Slowest);
         let s = DynamicSleeper::new(ScannerSpeed::Default);
@@ -362,7 +359,6 @@ mod tests {
     }
 
     #[tokio::test(start_paused = true)]
-    #[serial]
     async fn test_fastest_never_sleeps() {
         let prev_mode = SCANNER_IDLE_MODE.load(Ordering::Relaxed);
         SCANNER_IDLE_MODE.store(true, Ordering::Relaxed);
@@ -376,7 +372,6 @@ mod tests {
     }
 
     #[tokio::test(start_paused = true)]
-    #[serial]
     async fn test_idle_mode_off_skips_sleep() {
         let prev_mode = SCANNER_IDLE_MODE.load(Ordering::Relaxed);
         SCANNER_IDLE_MODE.store(false, Ordering::Relaxed);