docs: add MaxEventSubscribersPerSession to config shape example; assert its default

Add MaxEventSubscribersPerSession (value 8) to the Sessions block of the
Configuration Shape JSON example in GatewayConfiguration.md, matching the
appsettings.json default the options table already documents. Assert both
MaxEventSubscribersPerSession (8) and MaxPendingCommandsPerSession (128)
defaults in GatewayOptionsTests.OptionsBinding_UsesDesignDefaults.

docs/GatewayConfiguration.md

@@ -37,7 +37,8 @@ paths, timeouts, queue sizes, enum values, or protocol values are invalid.
       "MaxPendingCommandsPerSession": 128,
       "DefaultLeaseSeconds": 1800,
       "LeaseSweepIntervalSeconds": 30,
-      "AllowMultipleEventSubscribers": false
+      "AllowMultipleEventSubscribers": false,
+      "MaxEventSubscribersPerSession": 8
     },
     "Events": {
       "QueueCapacity": 10000,

src/ZB.MOM.WW.MxGateway.Tests/Configuration/GatewayOptionsTests.cs

@@ -31,9 +31,11 @@ public sealed class GatewayOptionsTests
 
         Assert.Equal(30, options.Sessions.DefaultCommandTimeoutSeconds);
         Assert.Equal(64, options.Sessions.MaxSessions);
+        Assert.Equal(128, options.Sessions.MaxPendingCommandsPerSession);
         Assert.Equal(1800, options.Sessions.DefaultLeaseSeconds);
         Assert.Equal(30, options.Sessions.LeaseSweepIntervalSeconds);
         Assert.False(options.Sessions.AllowMultipleEventSubscribers);
+        Assert.Equal(8, options.Sessions.MaxEventSubscribersPerSession);
 
         Assert.Equal(10_000, options.Events.QueueCapacity);
         Assert.Equal(EventBackpressurePolicy.FailFast, options.Events.BackpressurePolicy);