feat(sessions): detach-grace retention window for reconnect

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionManager.cs

@@ -17,6 +17,7 @@ public sealed class SessionManager : ISessionManager
     public const string DefaultCloseReason = "client-close";
     public const string GatewayShutdownReason = "gateway-shutdown";
     public const string LeaseExpiredReason = "lease-expired";
+    public const string DetachGraceExpiredReason = "detach-grace-expired";
 
     private readonly ISessionRegistry _registry;
     private readonly ISessionWorkerClientFactory _workerClientFactory;
@@ -295,12 +296,22 @@ public sealed class SessionManager : ISessionManager
         int closedCount = 0;
         foreach (GatewaySession session in _registry.Snapshot())
         {
-            if (!session.IsLeaseExpired(now))
+            // A session is swept when its normal lease has expired OR its detach-grace
+            // retention window has elapsed (last external subscriber dropped and no client
+            // reconnected within DetachGraceSeconds). The detach-grace close is the same
+            // teardown as a lease-expiry close; only the reason differs so operators can tell
+            // a short reconnect-window expiry from a long idle-lease expiry in logs/metrics.
+            string? reason = session.IsLeaseExpired(now)
+                ? LeaseExpiredReason
+                : session.IsDetachGraceExpired(now)
+                    ? DetachGraceExpiredReason
+                    : null;
+            if (reason is null)
             {
                 continue;
             }
 
-            await CloseSessionCoreAsync(session, LeaseExpiredReason, cancellationToken).ConfigureAwait(false);
+            await CloseSessionCoreAsync(session, reason, cancellationToken).ConfigureAwait(false);
             closedCount++;
         }
 
@@ -478,7 +489,8 @@ public sealed class SessionManager : ISessionManager
             shutdownTimeout,
             leaseDuration,
             openedAt,
-            eventStreaming);
+            eventStreaming,
+            TimeSpan.FromSeconds(Math.Max(0, _options.Sessions.DetachGraceSeconds)));
     }
 
     private static string CreateClientCorrelationId(