fix(sessions): make EventSubscriberLease dispose atomic; dedupe lease dispose

Issue 1: replace plain bool _disposed in EventSubscriberLease with an
Interlocked.Exchange int (_leaseDisposed) matching the SubscriberLease
pattern in SessionEventDistributor. Concurrent stream-completion +
client-cancellation racing Dispose() now decrements _activeEventSubscriberCount
exactly once, never to -1.

Issue 5: remove the `using` declaration on the subscriber lease in
EventStreamService.StreamEventsAsync; the finally block already disposes it
alongside the reader, so the using was a redundant second dispose on the
same code path.

Issue 2: add an inline comment at the StartAsync().GetAwaiter().GetResult()
call documenting the sync-over-async invariant (StartAsync only schedules via
Task.Run and is synchronous; do not make it truly async without changing
this call site).

Issue 10: remove the redundant .WithCancellation(cancellationToken) chained
on ReadEventsAsync(cancellationToken) in MapWorkerEventsAsync; the
[EnumeratorCancellation] token already flows through the direct argument.

Issue 9: add EventSubscriberLease_ConcurrentDispose_DecrementsCountExactlyOnce
to GatewaySessionTests — 16 concurrent Dispose() calls on the same lease for
200 iterations; asserts count is exactly 0 after each race and a subsequent
single-subscriber AttachEventSubscriber succeeds.

src/ZB.MOM.WW.MxGateway.Server/Grpc/EventStreamService.cs

@@ -53,7 +53,10 @@ public sealed class EventStreamService(
                 $"Session {request.SessionId} was not found.");
         }
 
-        using IEventSubscriberLease subscriber = session.AttachEventSubscriber(
+        // No `using` here — subscriber.Dispose() is called exactly once in the finally
+        // block below, which also disposes the reader. A `using` declaration would add a
+        // second Dispose on the same path and double-decrement the session subscriber count.
+        IEventSubscriberLease subscriber = session.AttachEventSubscriber(
             options.Value.Sessions.AllowMultipleEventSubscribers);
 
         int streamQueueDepth = 0;