fix(dashboard): close StartDashboardMirror/DisposeAsync race; internal-overflow test + metric label

(1) GatewaySession.StartDashboardMirror: publish _dashboardMirrorLease and _dashboardMirrorTask
    atomically under one _syncRoot section; if the session is already Closing/Closed/Faulted,
    dispose the just-created lease and return without starting the mirror task so nothing is orphaned.
(2) WaitUntilAsync test helper: catch OperationCanceledException and call Assert.Fail with the
    timeout duration and predicate source text instead of letting the exception propagate raw.
(3) New SessionEventDistributorTests.InternalSubscriberOverflow_HandlerSeesIsOnlySubscriberFalse:
    verifies CountExternalSubscribers excludes the internal subscriber, so isOnlySubscriber==false
    even when the internal subscriber is the only registered subscriber.
(4) SubscriberOverflowHandler delegate gains isInternal parameter; overflow metric label is
    "dashboard-mirror" for internal subscribers and "grpc-event-stream" for external ones.
(5) DashboardEventBroadcaster.Publish: wrap SendAsync Task acquisition in try/catch so a
    synchronous throw cannot escape the never-throw Publish interface contract.

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/GatewaySessionDashboardMirrorTests.cs

@@ -204,12 +204,19 @@ public sealed class GatewaySessionDashboardMirrorTests
         return new WorkerEvent { Event = mxEvent };
     }
 
-    private static async Task WaitUntilAsync(Func<bool> predicate)
+    private static async Task WaitUntilAsync(Func<bool> predicate, [CallerArgumentExpression(nameof(predicate))] string? condition = null)
     {
         using CancellationTokenSource cancellationTokenSource = new(TestTimeout);
-        while (!predicate())
+        try
         {
-            await Task.Delay(TimeSpan.FromMilliseconds(10), cancellationTokenSource.Token);
+            while (!predicate())
+            {
+                await Task.Delay(TimeSpan.FromMilliseconds(10), cancellationTokenSource.Token);
+            }
+        }
+        catch (OperationCanceledException)
+        {
+            Assert.Fail($"Timed out after {TestTimeout.TotalSeconds}s waiting for: {condition}");
         }
     }