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.Server/Dashboard/Hubs/DashboardEventBroadcaster.cs

@@ -24,9 +24,21 @@ public sealed class DashboardEventBroadcaster(
             return;
         }
 
-        Task send = hubContext.Clients
-            .Group(EventsHub.GroupName(sessionId))
-            .SendAsync(EventsHub.EventMessage, mxEvent);
+        // Wrap the Task acquisition in a try/catch so a hypothetical synchronous throw
+        // from SendAsync (e.g. an implementation that throws before returning the Task)
+        // cannot escape Publish. The interface contract is never-throw; fire-and-forget.
+        Task send;
+        try
+        {
+            send = hubContext.Clients
+                .Group(EventsHub.GroupName(sessionId))
+                .SendAsync(EventsHub.EventMessage, mxEvent);
+        }
+        catch (Exception ex)
+        {
+            logger.LogDebug(ex, "Dashboard event mirror to session {SessionId} threw synchronously.", sessionId);
+            return;
+        }
 
         if (!send.IsCompletedSuccessfully)
         {