fix(ilm): trace refused expiries with their error; harden the tests

The implementation review of this round's public fixes found one gap in
the code and four in the tests.

- A refused expiry was traced with its reason but without its error,
  so the trace did not say what refused it. It now carries the error, as
  a failed one does.
- The restore MRF test replaced the live MRF channel, which the heal
  routines of the test servers read: a race, and a consumer could take
  the entry under test. addPartialOp now offers an atomic hook that
  receives the operation in place of the queue. Production never sets
  it.
- Two fixtures restored the storage class with Update, which marks an
  uninitialized configuration initialized and so changes the default
  parity (-1) into an explicit 0 for later tests.
  setTestStandardParity restores the whole saved value under
  storageclass.ConfigLock.
- The v3 metric test built its own descriptor map, so dropping the
  counter from the registered ILM collector still passed. It gathers
  through the collector the handler registers.
- The audit test read one extra entry or waited 500 ms. It drains every
  entry of the bucket up to the removal audited last, and checks one
  failure audit, one removal audit and no refusal audit per object.

Each corrected test fails with the corresponding mutation.

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

cmd/mrf.go

@@ -74,11 +74,20 @@ func newMRFState() mrfState {
 	}
 }
 
+// mrfPartialOpHook, when set, receives each partial operation in place of
+// the queue, so that a test observes what is queued without racing the heal
+// routines that consume it. Production never sets it.
+var mrfPartialOpHook atomic.Pointer[func(PartialOperation)]
+
 // Add a partial S3 operation (put/delete) when one or more disks are offline.
 func (m *mrfState) addPartialOp(op PartialOperation) {
 	if m == nil {
 		return
 	}
+	if f := mrfPartialOpHook.Load(); f != nil {
+		(*f)(op)
+		return
+	}
 
 	if atomic.LoadInt32(&m.closed) == 1 {
 		return