fix(opcuaclient): an acknowledge with no reply fails with its transport status (R06:O-8)

AcknowledgeAsync swallowed every non-cancellation transport failure (and returned
normally when the session was lost while the ack queued), so the host logged the
Part 9 ack as applied while upstream still showed it unacknowledged. Both now throw
(ServiceResultException with the transport status / BadNotConnected). A reply,
including per-ack refusals, still returns normally and counts as the server answering.

Refs #743

Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

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

@@ -2496,9 +2496,12 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
             // Re-read Session inside the critical section: a reconnect completing while we were
             // blocked on _gate has already swapped in the new session via a re-initialise.
             // ConditionId parsing is namespace-relative, so it must use the current session's
-            // namespace table too.
-            var session = Session;
-            if (session is null) return;
+            // namespace table too. A session lost while this ack queued is a failure, not a silent success: the
+            // host would otherwise log the ack as applied (R06:O-8).
+            var generation = Volatile.Read(ref _lifecycleGeneration);
+            var session = Session
+                ?? throw new ServiceResultException(StatusCodes.BadNotConnected,
+                    "OpcUaClient session was lost before the acknowledgement could be sent.");
 
             // OPC UA A&C: call the AcknowledgeableConditionType.Acknowledge method on each
             // condition node with EventId + Comment arguments. CallAsync accepts a batch —
@@ -2543,6 +2546,8 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
                     requestHeader: null,
                     methodsToCall: callRequests,
                     ct: cancellationToken).ConfigureAwait(false);
+                // A response — whatever the per-ack statuses say — is the server answering.
+                RecordDeviceAnswered(generation, Clock.GetUtcNow().UtcDateTime);
 
                 // Inspect per-ack results — the upstream server can reject individual acks
                 // (BadConditionAlreadyAcked, BadNodeIdUnknown, BadUserAccessDenied) even when
@@ -2575,11 +2580,13 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
             }
             catch (Exception ex)
             {
-                // Log genuine transport failures rather than swallowing them silently.
+                // R06:O-8 — a transport failure means NO reply: the acknowledgement was not applied upstream, so the
+                // caller must see it (with the transport status, e.g. BadConnectionClosed) instead of a success.
                 _logger.LogWarning(ex,
                     "OpcUaClientDriver '{DriverInstanceId}': AcknowledgeAsync failed; " +
-                    "acknowledgements may not have been applied.",
+                    "acknowledgements were not applied.",
                     _driverInstanceId);
+                throw;
             }
         }
         finally { _gate.Release(); }