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.

src/Client/ZB.MOM.WW.OtOpcUa.Client.Shared/Adapters/DefaultSessionAdapter.cs

@@ -108,7 +108,9 @@ internal sealed class DefaultSessionAdapter : ISessionAdapter
     public async Task<(byte[]? ContinuationPoint, ReferenceDescriptionCollection References)> BrowseNextAsync(
         byte[] continuationPoint, CancellationToken ct)
     {
-        var (_, nextCp, nextRefs) = await _session.BrowseNextAsync(null, false, continuationPoint);
+        // Pass the caller's token so a cancelled browse does not block on the continuation
+        // round-trip (Client.Shared-015).
+        var (_, nextCp, nextRefs) = await _session.BrowseNextAsync(null, false, continuationPoint, ct);
         return (nextCp, nextRefs ?? []);
     }