review(Client.Shared): fix Disconnect/failover subscription race + CT forwarding

Re-review at 7286d320. -012 (Medium): DisconnectAsync now snapshots+nulls the data/alarm
subscriptions under _subscriptionLock before async teardown (was racing RunFailoverAsync).
-013: SubscribeAlarmsAsync guarded by a semaphore (idempotent under concurrency). -014/-015:
forward CancellationToken through Delete/BrowseNext adapters. + TDD.

tests/Client/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/Fakes/FakeSubscriptionAdapter.cs

@@ -101,10 +101,17 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
         return Task.CompletedTask;
     }
 
+    /// <summary>
+    /// Gets the cancellation token that was supplied to the most recent <see cref="DeleteAsync"/> call,
+    /// so tests can assert the CT from the caller is honoured (Client.Shared-014).
+    /// </summary>
+    public CancellationToken? LastDeleteCt { get; private set; }
+
     /// <inheritdoc />
     public Task DeleteAsync(CancellationToken ct)
     {
         Deleted = true;
+        LastDeleteCt = ct;
         lock (_itemsLock) _items.Clear();
         return Task.CompletedTask;
     }