refactor(sessions): derive subscriber mode from session config; close Task 8 review nits

Remove the per-call allowMultipleSubscribers param from AttachEventSubscriber and
derive the mode internally from _eventStreaming.AllowMultipleEventSubscribers — the
same source SessionEventDistributor uses for singleSubscriberMode — so the two can
never structurally diverge. The maxSubscribers cap param is kept because
MaxEventSubscribersPerSession lives in SessionOptions, which the session does not hold
directly (only EventOptions flows through SessionEventStreaming).

Other nits:
- SubscriberCount XML doc clarifies it includes internal subscribers and differs from
  GatewaySession.ActiveEventSubscriberCount (external/gRPC only).
- SingleSubscriberMode_LoneExternalOverflow test: add Assert.Equal(1, observedSet) guard
  before the value assertion so the test cannot pass vacuously if the handler never fired.
- GatewayOptionsValidator.ValidateSessions: add explanatory code comment documenting why
  !AllowMultipleEventSubscribers && MaxEventSubscribersPerSession > 1 is NOT rejected as
  a hard error (the default config ships with this combination; the cap is simply unused
  in single-subscriber mode, not a behavior bug).
- GatewaySession.DetachEventSubscriber: add Debug.Assert before the clamp so a genuine
  double-decrement surfaces in debug builds.

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

@@ -68,8 +68,10 @@ public sealed class EventStreamService(
         // No `using` here — subscriber.Dispose() is called exactly once in the finally
         // 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.
+        // The subscriber mode (single vs. multi) is derived inside AttachEventSubscriber from
+        // the session's own SessionEventStreaming.AllowMultipleEventSubscribers field — the
+        // same source the distributor uses — so the two cannot diverge.
         IEventSubscriberLease subscriber = session.AttachEventSubscriber(
-            options.Value.Sessions.AllowMultipleEventSubscribers,
             options.Value.Sessions.MaxEventSubscribersPerSession);
 
         int streamQueueDepth = 0;