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.Tests/Gateway/Sessions/SessionEventDistributorTests.cs

@@ -113,6 +113,27 @@ public sealed class SessionEventDistributorTests
         Assert.Throws<ObjectDisposedException>(() => distributor.Register());
     }
 
+    [Fact]
+    public async Task RegisterWithReplay_AfterDispose_ThrowsObjectDisposedException()
+    {
+        // Pins the nested-lock disposal behavior in RegisterWithReplay: the inner
+        // _lifecycleLock check must surface ObjectDisposedException even when the outer
+        // _replayLock snapshot succeeds on a disposed distributor.
+        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
+        SessionEventDistributor distributor = CreateDistributor(source.Reader);
+        await distributor.StartAsync(CancellationToken.None);
+
+        await distributor.DisposeAsync().AsTask().WaitAsync(ReadTimeout);
+
+        Assert.Throws<ObjectDisposedException>(() =>
+            distributor.RegisterWithReplay(
+                0,
+                out _,
+                out _,
+                out _,
+                out _));
+    }
+
     [Fact]
     public async Task ReplayBuffer_OverCapacity_EvictsOldestFirst_AndReportsGap()
     {
@@ -601,7 +622,8 @@ public sealed class SessionEventDistributorTests
         Assert.False(gap);
         Assert.Equal(new ulong[] { 3, 4, 5 }, replay.Select(e => e.WorkerSequence));
         Assert.Equal(5ul, liveResume);
-        Assert.Equal(1ul, oldestAvailable);
+        // OldestAvailableSequence is 0 when gap == false (meaningful only when gap is true).
+        Assert.Equal(0ul, oldestAvailable);
 
         // A subsequent live event flows to the resumed subscriber's channel.
         source.Writer.TryWrite(Event(6));
@@ -669,7 +691,8 @@ public sealed class SessionEventDistributorTests
         Assert.False(gap);
         Assert.Empty(replay);
         Assert.Equal(3ul, liveResume);
-        Assert.Equal(1ul, oldestAvailable);
+        // OldestAvailableSequence is 0 when gap == false (meaningful only when gap is true).
+        Assert.Equal(0ul, oldestAvailable);
 
         source.Writer.TryWrite(Event(4));
         MxEvent live = await ReadOneAsync(resume.Reader);