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/EffectiveSessionConfiguration.cs

@@ -6,4 +6,5 @@ public sealed record EffectiveSessionConfiguration(
     int MaxPendingCommandsPerSession,
     int DefaultLeaseSeconds,
     int LeaseSweepIntervalSeconds,
-    bool AllowMultipleEventSubscribers);
+    bool AllowMultipleEventSubscribers,
+    int MaxEventSubscribersPerSession);