fix(sessions): expose DetachGraceSeconds in effective-config; single clock; close reconnect-vs-sweep race

- EffectiveSessionConfiguration: add DetachGraceSeconds field; GatewayConfigurationProvider
  forwards value.Sessions.DetachGraceSeconds (blocker fix).
- GatewaySession.InvokeAsync and ReadEventsAsync: switch TouchClientActivity calls from
  DateTimeOffset.UtcNow to _eventStreaming.TimeProvider.GetUtcNow() so Task 12 fake-clock
  control works end-to-end (split-clock fix).
- TOCTOU fix: add TryBeginCloseIfExpired(now, out alreadyClosing) to GatewaySession that
  re-checks IsLeaseExpiredCore/IsDetachGraceExpiredCore AND _activeEventSubscriberCount==0
  under _syncRoot before transitioning to Closing; CloseExpiredLeasesAsync calls it before
  CloseSessionCoreAsync so a reattach that wins the race leaves the session Ready/usable.
- Minors: lease-expiry-takes-precedence comment in CloseExpiredLeasesAsync; TOCTOU comment
  block; sweep-cycle latency note added to SessionOptions.DetachGraceSeconds XML doc and to
  GatewayConfiguration.md DetachGraceSeconds row.
- New tests: TryBeginCloseIfExpired_ReattachedSubscriberWinsRace_DeclinesClose (GatewaySession),
  CloseExpiredLeasesAsync_DoesNotCloseSessionThatReattachedBeforeSweepCloses (SessionManager),
  plus IsLeaseExpiredCore/IsDetachGraceExpiredCore private helpers used by the guard.

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/GatewaySessionTests.cs

@@ -577,6 +577,43 @@ public sealed class GatewaySessionTests
         Assert.True(session.IsDetachGraceExpired(clock.GetUtcNow()));
     }
 
+    /// <summary>
+    ///     Task 11. Validates the TOCTOU fix: TryBeginCloseIfExpired atomically re-checks that no
+    ///     subscriber has reattached before flipping to Closing. When the grace window has elapsed but
+    ///     a subscriber is attached by the time TryBeginCloseIfExpired runs, it returns false and the
+    ///     session remains Ready.
+    /// </summary>
+    [Fact]
+    public async Task TryBeginCloseIfExpired_ReattachedSubscriberWinsRace_DeclinesClose()
+    {
+        FakeWorkerClient workerClient = new();
+        FakeTimeProvider clock = new(DateTimeOffset.UtcNow);
+        await using GatewaySession session = CreateReadySessionWithDetachGrace(
+            workerClient,
+            clock,
+            detachGrace: TimeSpan.FromSeconds(30));
+
+        // Attach then drop to enter detach-grace, then advance past the window.
+        IDisposable firstSubscriber = session.AttachEventSubscriber(maxSubscribers: 1);
+        firstSubscriber.Dispose();
+        clock.Advance(TimeSpan.FromSeconds(31));
+        DateTimeOffset expiredNow = clock.GetUtcNow();
+        Assert.True(session.IsDetachGraceExpired(expiredNow)); // sanity: would be closed by sweep
+
+        // Simulate a client reconnecting before the sweeper calls TryBeginCloseIfExpired.
+        // The reattach clears _detachedAtUtc and increments _activeEventSubscriberCount so
+        // neither expiry condition holds any longer.
+        using IDisposable reconnected = session.AttachEventSubscriber(maxSubscribers: 1);
+        Assert.Null(session.DetachedAtUtc);
+
+        // TryBeginCloseIfExpired must see the reattach and decline — the session stays Ready.
+        bool began = session.TryBeginCloseIfExpired(expiredNow, out bool alreadyClosing);
+
+        Assert.False(began);
+        Assert.False(alreadyClosing);
+        Assert.Equal(SessionState.Ready, session.State);
+    }
+
     private static GatewaySession CreateReadySessionWithDetachGrace(
         IWorkerClient workerClient,
         TimeProvider timeProvider,