test(opcuaclient): slow server-side subscription delete keeps the stale-BadNoSubscription grace running from delete completion (lmxopcua#764)

Unsubscribe paths now delete through the existing DeleteSubscriptionAsync seam so a test can hold the delete open >grace. Mutation: dropping the post-delete restamp fails both slow-delete tests.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -1242,7 +1242,7 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
     /// </summary>
     internal TimeSpan TeardownBudget { get; set; } = TimeSpan.FromSeconds(4);
 
-    /// <summary>Test seam: the wire delete of one subscription. Production calls the SDK's silent delete.</summary>
+    /// <summary>Test seam: the wire delete of one subscription (unsubscribe + shutdown). Production calls the SDK's silent delete.</summary>
     internal Func<Subscription, CancellationToken, Task> DeleteSubscriptionAsync { get; set; } =
         static (subscription, ct) => subscription.DeleteAsync(silent: true, ct);
 
@@ -2050,7 +2050,7 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
             // RemoteSubscription is the exact instance that was added, so `-=` removes it
             // cleanly.
             DetachNotificationHandlers(rs.ItemHandlers);
-            try { await rs.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
+            try { await DeleteSubscriptionAsync(rs.Subscription, cancellationToken).ConfigureAwait(false); }
             catch { /* best-effort — the subscription may already be gone on reconnect */ }
         }
         finally
@@ -2555,7 +2555,7 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
             // closure.
             try { rs.EventItem.Notification -= rs.Handler; }
             catch { /* best-effort */ }
-            try { await rs.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
+            try { await DeleteSubscriptionAsync(rs.Subscription, cancellationToken).ConfigureAwait(false); }
             catch { /* best-effort — session may already be gone across a reconnect */ }
         }
         finally