fix(notify): close core notify correctness and safety gaps (#4502)

Land the remaining notify-crate audit fixes.

backlog#979(b): remove_target now enforces the same bucket-binding guard as
remove_target_config, refusing to delete a target still referenced by a bucket
rule so notification rules are not left orphaned.

backlog#984:
- event.rs: an unversioned object omits versionId entirely instead of
  serializing versionId:"" (empty object/request versions treated as "no
  version").
- notifier.rs: RUSTFS_NOTIFY_SEND_CONCURRENCY=0 coerces back to the default
  instead of building a zero-permit semaphore that deadlocks every dispatch;
  init_bucket_targets_shared closes the replaced targets instead of dropping
  them without close() (connection leak).
- subscriber_index.rs: store_snapshot uses an atomic compute_if_absent upsert,
  removing the get-then-insert TOCTOU that could clobber a concurrent
  first-writer's snapshot cell.
- pipeline.rs: send_event assigns the history sequence and broadcasts to live
  subscribers under one critical section so broadcast order matches recorded
  sequence order.
- xml_config.rs: filter value length is bounded by character count, not byte
  length, so valid multi-byte keys are no longer wrongly rejected.
- global.rs: a losing initialize() race shuts the just-initialized system down
  instead of leaking its targets/replay workers.

backlog#970 (notify part): reload_config stops the running replay workers
before activating the new ones, so old and new workers do not concurrently
drain the same persisted stores. The full signal+join shutdown lives in the
targets crate under the same issue.

Tests: added regression coverage for each fix.
cargo build -p rustfs-notify, cargo test -p rustfs-notify --lib (98 passed),
cargo clippy -p rustfs-notify --all-targets (clean).

Relates to rustfs/backlog#979
Relates to rustfs/backlog#984
Relates to rustfs/backlog#970

Co-authored-by: heihutu <heihutu@gmail.com>

crates/notify/src/global.rs

@@ -37,7 +37,14 @@ pub async fn initialize(config: Config) -> Result<(), NotificationError> {
 
     match NOTIFICATION_SYSTEM.set(Arc::new(system)) {
         Ok(_) => Ok(()),
-        Err(_) => Err(NotificationError::Lifecycle(LifecycleError::AlreadyInitialized)),
+        Err(losing_system) => {
+            // Another initializer won the race. `init()` above already started this
+            // system's targets and replay workers, so simply dropping it would leak
+            // those background tasks. Shut the losing instance down cleanly before
+            // reporting the conflict (backlog#984).
+            losing_system.shutdown().await;
+            Err(NotificationError::Lifecycle(LifecycleError::AlreadyInitialized))
+        }
     }
 }