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/Grpc/EventStreamService.cs

@@ -132,16 +132,12 @@ public sealed class EventStreamService(
 
             foreach (MxEvent replayedEvent in replayedEvents)
             {
-                // Replayed events pass through the SAME per-item filter the live loop applies,
-                // so a constrained/resuming caller never sees a replayed event it could not
-                // have seen live. The watermark dropped events at/below the requested
-                // AfterWorkerSequence; the snapshot already excluded those, but this keeps the
-                // filter identical for replay and live.
-                if (replayedEvent.WorkerSequence <= request.AfterWorkerSequence)
-                {
-                    continue;
-                }
-
+                // RegisterWithReplay already returns only events strictly newer than
+                // AfterWorkerSequence, so no per-item sequence guard is needed here.
+                // There is no per-event constraint filter on the event stream: events are
+                // fanned as-is by the distributor pump. The only dedup watermark is the
+                // LiveResumeSequence applied in the live loop below (to drop any event
+                // that was both replayed and raced into the live channel).
                 yield return replayedEvent;
             }