Joseph Doherty
2ead9bc200
(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.