feat(config): allow multiple event subscribers + add MaxEventSubscribersPerSession cap

Remove the hard-rejection of AllowMultipleEventSubscribers=true in GatewayOptionsValidator
(fan-out is now implemented via SessionEventDistributor). Add MaxEventSubscribersPerSession
(default 8, must be >= 1) to SessionOptions, validate it, expose it in
EffectiveSessionConfiguration / GatewayConfigurationProvider, document it in
GatewayConfiguration.md and appsettings.json. Tests cover the no-error path for
AllowMultipleEventSubscribers=true, the 0/-1 rejection, positive pass, and default pass.

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayOptionsValidator.cs

@@ -177,12 +177,10 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
             options.LeaseSweepIntervalSeconds,
             "MxGateway:Sessions:LeaseSweepIntervalSeconds must be greater than zero.",
             builder);
-
-        if (options.AllowMultipleEventSubscribers)
-        {
-            builder.Add(
-                "MxGateway:Sessions:AllowMultipleEventSubscribers is not supported until event fan-out is implemented.");
-        }
+        AddIfNotPositive(
+            options.MaxEventSubscribersPerSession,
+            "MxGateway:Sessions:MaxEventSubscribersPerSession must be greater than zero.",
+            builder);
     }
 
     private static void ValidateEvents(EventOptions options, ValidationBuilder builder)