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/e2e_test/src/reliability_disk_fault_test.rs

@@ -27,7 +27,6 @@ mod tests {
     use aws_sdk_s3::Client;
     use aws_sdk_s3::primitives::ByteStream;
     use aws_sdk_s3::types::{BucketVersioningStatus, CompletedMultipartUpload, CompletedPart, VersioningConfiguration};
-    use serial_test::serial;
     use sha2::{Digest, Sha256};
     use std::collections::HashSet;
     use std::error::Error;
@@ -157,7 +156,6 @@ mod tests {
     /// content, degraded writes must succeed, and everything must still
     /// verify after the disk returns.
     #[tokio::test]
-    #[serial]
     async fn test_degraded_read_write_with_one_disk_offline() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
         init_logging();
         info!("Reliability: degraded read/write with one of four disks offline");
@@ -210,7 +208,6 @@ mod tests {
     /// bytes to a reader: per-shard bitrot checksums reject the bad shard and
     /// the object is reconstructed from the remaining shards.
     #[tokio::test]
-    #[serial]
     async fn test_bitrot_corrupted_shard_read_returns_correct_data() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
         init_logging();
         info!("Reliability: GET must read through a bitrot-corrupted shard");
@@ -253,7 +250,6 @@ mod tests {
     /// heal, and require the replaced disk to be rebuilt and all content to
     /// verify against the sha256 manifest.
     #[tokio::test]
-    #[serial]
     async fn test_fresh_disk_replacement_heals_after_sigkill_restart() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
         init_logging();
         info!("Reliability: fresh-disk replacement heals after SIGKILL restart");
@@ -327,7 +323,6 @@ mod tests {
     }
 
     #[tokio::test]
-    #[serial]
     async fn test_versioned_shard_census_selects_each_version_data_dir() -> Result<(), Box<dyn Error + Send + Sync>> {
         init_logging();
         info!("Reliability: physical shard census selects the requested object version");