test(pools): make the copy and bulk lock tests race-free and stricter

The implementation review of the lock fixes found test flaws, which are
corrected here; the production behavior is unchanged.

- The alias and merge tests replaced globalOperationTimeout while
  background healing reads it, a data race under -race. They now bound
  each request with its own context instead.
- The alias test now holds the cleaned name's write lock while the copy
  runs, so it proves the names are checked before anything is locked;
  it checks both objects before any later write, and covers a/./x as a
  destination.
- The merge test picks aliases that hash to the clean name's set and,
  where sets have their own lockers, ones that do not, and requires the
  alias's own table to hold a read lock: readers admitted, writers
  excluded.
- Release polling creates a fresh lock instance for every probe.
- The getNSLocks event hook and the bulk delete group limit are atomic,
  so tests can set them while other goroutines read them.

Without the name check and the resource merge, both tests now fail in
every lock layout.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/erasure-server-pool-lock-bulk_test.go

@@ -122,9 +122,8 @@ func TestDeleteObjectsInSetsRunsGroupsConcurrently(t *testing.T) {
 // the result its own delete reports, and a group that completed before keeps
 // its success.
 func TestDeleteObjectsInSetsQueuedGroupsCancel(t *testing.T) {
-	prev := deleteSetGroupsLimit
-	deleteSetGroupsLimit = 1
-	t.Cleanup(func() { deleteSetGroupsLimit = prev })
+	deleteSetGroupsLimit.Store(1)
+	t.Cleanup(func() { deleteSetGroupsLimit.Store(0) })
 	for _, blocked := range []int{0, 1} {
 		t.Run(fmt.Sprintf("blocked=%d", blocked), func(t *testing.T) {
 			z, bucket := bulkDeletePools(t, 1)