refactor(driver-pages): address Phase 6/8 deep-review findings

- Topic-name drift fix: DriverHealthChanged.TopicName and
  DriverControlTopic.Name now live on the message contracts in
  Commons. AkkaDriverHealthPublisher, DriverStatusSignalRBridge,
  DriverHostActor, and AdminOperationsActor all delegate to the
  single constant so a rename can't silently desynchronise
  publisher and subscriber.
- DriverStatusPanel._opResultClearTimer switched from
  System.Timers.Timer to System.Threading.Timer + awaited
  DisposeAsync. Prevents an in-flight 8s clear-callback from
  invoking StateHasChanged on a component whose hub has already
  been released.
- PublishHealthSnapshot deduplicates against the last published
  (state, lastSuccess, lastError, errorCount) fingerprint. The
  30s heartbeat no longer floods the SignalR layer with identical
  Healthy snapshots — newly-joined clients still warm up via the
  snapshot store on JoinDriver.

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Drivers/DriverStatusPanel.razor

@@ -157,7 +157,7 @@
     private bool _showRestartConfirm;
     private string? _opResultMessage;
     private bool _opResultOk;
-    private System.Timers.Timer? _opResultClearTimer;
+    private System.Threading.Timer? _opResultClearTimer;
 
     protected override async Task OnInitializedAsync()
     {
@@ -257,24 +257,23 @@
     {
         _opResultOk = ok;
         _opResultMessage = message;
-        // Auto-clear the result chip after 8 s.
+        // Auto-clear the result chip after 8 s. System.Threading.Timer is used (not
+        // System.Timers.Timer) so DisposeAsync can drain any in-flight callback.
         _opResultClearTimer?.Dispose();
-        _opResultClearTimer = new System.Timers.Timer(8_000) { AutoReset = false };
-        _opResultClearTimer.Elapsed += async (_, _) =>
+        _opResultClearTimer = new System.Threading.Timer(_ =>
         {
             _opResultMessage = null;
-            await InvokeAsync(StateHasChanged);
-        };
-        _opResultClearTimer.Start();
+            InvokeAsync(StateHasChanged);
+        }, null, TimeSpan.FromSeconds(8), Timeout.InfiniteTimeSpan);
     }
 
     public async ValueTask DisposeAsync()
     {
-        // Drain the timer first so an in-flight callback can't invoke StateHasChanged on
-        // a component that's already releasing its hub. System.Threading.Timer implements
-        // IAsyncDisposable in .NET 6+; the async dispose awaits any in-flight callback.
+        // Drain BOTH timers first so an in-flight callback can't invoke StateHasChanged on
+        // a component whose hub has already been released. System.Threading.Timer's async
+        // dispose awaits any in-flight callback (.NET 6+).
         if (_timer is not null) await _timer.DisposeAsync();
-        _opResultClearTimer?.Dispose();
+        if (_opResultClearTimer is not null) await _opResultClearTimer.DisposeAsync();
         if (_hub is not null) await _hub.DisposeAsync();
     }