feat(server): add MxGateway:Sessions:WorkerReadyWaitTimeoutMs (default off)

Adds WorkerReadyWaitTimeoutMs to SessionOptions (default 0 = disabled),
validates >= 0 in GatewayOptionsValidator, documents it in
GatewayConfiguration.md, and adds validator + default-value tests.
No wait/poll logic is implemented here (that is Task 8).

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

@@ -185,6 +185,10 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
             options.DetachGraceSeconds,
             "MxGateway:Sessions:DetachGraceSeconds must be zero or greater (0 disables detach-grace retention).",
             builder);
+        AddIfNegative(
+            options.WorkerReadyWaitTimeoutMs,
+            "MxGateway:Sessions:WorkerReadyWaitTimeoutMs must be greater than or equal to zero.",
+            builder);
 
         // NOTE: We intentionally do NOT reject !AllowMultipleEventSubscribers &&
         // MaxEventSubscribersPerSession > 1 as a hard validation error here. The default