fix(historian-gateway): guard canceled antecedent in provisioning continuation

Addresses T15 review: treat a canceled EnsureTags task like a faulted one so the
fire-and-forget continuation never reaches t.Result (which would re-throw and leave
the discarded task unobserved).

Claude-Session: https://claude.ai/code/session_012SDSQ3AcaXqPcBtDESBRii

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/AddressSpaceApplier.cs

@@ -246,10 +246,12 @@ public sealed class AddressSpaceApplier
             _ = dispatch.ContinueWith(
                 t =>
                 {
-                    if (t.IsFaulted)
+                    if (!t.IsCompletedSuccessfully)
                     {
+                        // Faulted OR canceled — never reach t.Result (which would re-throw and
+                        // leave this discarded continuation unobserved).
                         _logger.LogWarning(t.Exception?.GetBaseException(),
-                            "AddressSpaceApplier: historian provisioning of {Count} tag(s) faulted; deploy unaffected", provisionCount);
+                            "AddressSpaceApplier: historian provisioning of {Count} tag(s) did not complete; deploy unaffected", provisionCount);
                         return;
                     }