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.Server/Configuration/EffectiveSessionConfiguration.cs

@@ -6,5 +6,6 @@ public sealed record EffectiveSessionConfiguration(
     int MaxPendingCommandsPerSession,
     int DefaultLeaseSeconds,
     int LeaseSweepIntervalSeconds,
+    int DetachGraceSeconds,
     bool AllowMultipleEventSubscribers,
     int MaxEventSubscribersPerSession);

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayConfigurationProvider.cs

@@ -46,6 +46,7 @@ public sealed class GatewayConfigurationProvider(IOptions<GatewayOptions> option
                 MaxPendingCommandsPerSession: value.Sessions.MaxPendingCommandsPerSession,
                 DefaultLeaseSeconds: value.Sessions.DefaultLeaseSeconds,
                 LeaseSweepIntervalSeconds: value.Sessions.LeaseSweepIntervalSeconds,
+                DetachGraceSeconds: value.Sessions.DetachGraceSeconds,
                 AllowMultipleEventSubscribers: value.Sessions.AllowMultipleEventSubscribers,
                 MaxEventSubscribersPerSession: value.Sessions.MaxEventSubscribersPerSession),
             Events: new EffectiveEventConfiguration(

src/ZB.MOM.WW.MxGateway.Server/Configuration/SessionOptions.cs

@@ -36,6 +36,13 @@ public sealed class SessionOptions
     ///     expires. The reconnect/replay itself is implemented separately (Task 12); this
     ///     option controls retention and expiry only.
     /// </summary>
+    /// <remarks>
+    ///     The effective close happens within the next sweep cycle after the window elapses —
+    ///     up to <see cref="LeaseSweepIntervalSeconds"/> after expiry. Operators who want a
+    ///     firm minimum bound should set <c>DetachGraceSeconds</c> greater than
+    ///     <see cref="LeaseSweepIntervalSeconds"/>; otherwise a session whose window expires
+    ///     just before a sweep run may be closed within seconds of detach.
+    /// </remarks>
     public int DetachGraceSeconds { get; init; } = 30;
 
     /// <summary>