feat(sessions): multi-subscriber cap enforcement + mode-gated FailFast

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionEventStreaming.cs

@@ -38,13 +38,24 @@ namespace ZB.MOM.WW.MxGateway.Server.Sessions;
 ///     EventsHub group regardless of whether a gRPC client is streaming. When null
 ///     (unit tests that don't exercise the dashboard mirror) no mirror is started.
 /// </param>
+/// <param name="AllowMultipleEventSubscribers">
+///     The session's effective multi-subscriber mode (Task 8). Carried here so the session
+///     can pass it to its <see cref="SessionEventDistributor"/> at construction — the
+///     distributor is created at <c>MarkReady</c> (for the dashboard mirror) before any gRPC
+///     subscriber attaches, so the mode cannot be learned from a later
+///     <c>AttachEventSubscriber</c> call. The distributor gates its FailFast session-fault
+///     decision on this mode (single-subscriber only) instead of a live count snapshot,
+///     closing the Task 5 false-FailFast race. Defaults to <see langword="false"/>
+///     (single-subscriber) so existing call sites and unit tests are unchanged.
+/// </param>
 public sealed record SessionEventStreaming(
     MxAccessGrpcMapper Mapper,
     EventOptions EventOptions,
     ILogger<SessionEventDistributor> DistributorLogger,
     TimeProvider TimeProvider,
     GatewayMetrics Metrics,
-    IDashboardEventBroadcaster? DashboardBroadcaster = null)
+    IDashboardEventBroadcaster? DashboardBroadcaster = null,
+    bool AllowMultipleEventSubscribers = false)
 {
     /// <summary>
     ///     Defaults used when a session is constructed without explicit streaming