fix(sessions): tidy replay filter/comment; zero OldestAvailableSequence when no gap

- EventStreamService: remove dead per-item sequence guard in the replay
  loop (RegisterWithReplay already returns only events > afterSequence)
  and correct the comment that falsely claimed a "per-item constraint
  filter" is applied; the event stream has no per-event constraint
  filtering today.
- SessionEventDistributor.RegisterWithReplay: set oldestAvailableSequence=0
  when gap==false so the implementation matches the documented contract
  (OldestAvailableSequence is meaningful only when Gap is true).
  Update the two RegisterWithReplay tests that asserted the old non-zero
  value in the no-gap path.
- RegisterSubscriber: remove stray blank line at method entry.
- SessionEventDistributorTests: add RegisterWithReplay_AfterDispose_
  ThrowsObjectDisposedException to pin nested-lock disposal behavior.

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionEventDistributor.cs

@@ -295,7 +295,6 @@ public sealed class SessionEventDistributor : IAsyncDisposable
 
     private IEventSubscriberLease RegisterSubscriber(Subscriber subscriber)
     {
-
         // The disposed check AND the map add happen under the same lock with no await
         // in between. DisposeAsync sets _disposed=true under this same lock before it
         // calls CompleteAllSubscribers, so once disposal has begun no further subscriber
@@ -424,13 +423,15 @@ public sealed class SessionEventDistributor : IAsyncDisposable
 
             if (_replayBuffer.Count == 0)
             {
-                oldestAvailableSequence = 0;
                 gap = _anyEventSeen && afterSequence < _highestSequenceSeen;
+                oldestAvailableSequence = 0; // meaningful only when gap == true; 0 here since nothing is retained
             }
             else
             {
-                oldestAvailableSequence = _replayBuffer.First!.Value.Event.WorkerSequence;
-                gap = oldestAvailableSequence > 0 && afterSequence < oldestAvailableSequence - 1;
+                ulong oldestRetained = _replayBuffer.First!.Value.Event.WorkerSequence;
+                gap = oldestRetained > 0 && afterSequence < oldestRetained - 1;
+                // Per the contract on OldestAvailableSequence: meaningful only when gap == true.
+                oldestAvailableSequence = gap ? oldestRetained : 0;
 
                 foreach (ReplayEntry entry in _replayBuffer)
                 {