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

src/ZB.MOM.WW.MxGateway.Server/Grpc/EventStreamService.cs

@@ -69,7 +69,8 @@ public sealed class EventStreamService(
         // block below, which also disposes the reader. A `using` declaration would add a
         // second Dispose on the same path and double-decrement the session subscriber count.
         IEventSubscriberLease subscriber = session.AttachEventSubscriber(
-            options.Value.Sessions.AllowMultipleEventSubscribers);
+            options.Value.Sessions.AllowMultipleEventSubscribers,
+            options.Value.Sessions.MaxEventSubscribersPerSession);
 
         int streamQueueDepth = 0;
         ulong afterWorkerSequence = request.AfterWorkerSequence;

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -949,6 +949,7 @@ public sealed class MxAccessGatewayService(
             SessionManagerErrorCode.SessionNotFound => StatusCode.NotFound,
             SessionManagerErrorCode.SessionNotReady => StatusCode.FailedPrecondition,
             SessionManagerErrorCode.EventSubscriberAlreadyActive => StatusCode.ResourceExhausted,
+            SessionManagerErrorCode.EventSubscriberLimitReached => StatusCode.ResourceExhausted,
             SessionManagerErrorCode.EventQueueOverflow => StatusCode.ResourceExhausted,
             SessionManagerErrorCode.SessionLimitExceeded => StatusCode.ResourceExhausted,
             SessionManagerErrorCode.OpenFailed => StatusCode.Unavailable,