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.Tests/Configuration/GatewayOptionsValidatorTests.cs

@@ -356,4 +356,41 @@ public sealed class GatewayOptionsValidatorTests
         ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
         Assert.True(result.Succeeded);
     }
+
+    // -------------------------------------------------------------------------
+    // WorkerReadyWaitTimeoutMs validation
+    // -------------------------------------------------------------------------
+
+    [Fact]
+    public void Validate_Fails_WhenWorkerReadyWaitTimeoutMsIsNegative()
+    {
+        GatewayOptions options = CloneWithSessions(
+            ValidOptions(),
+            new SessionOptions { WorkerReadyWaitTimeoutMs = -1 });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Failed);
+        Assert.Contains(
+            result.Failures!,
+            f => f.Contains("MxGateway:Sessions:WorkerReadyWaitTimeoutMs"));
+    }
+
+    [Fact]
+    public void Validate_Succeeds_WhenWorkerReadyWaitTimeoutMsIsZero()
+    {
+        GatewayOptions options = CloneWithSessions(
+            ValidOptions(),
+            new SessionOptions { WorkerReadyWaitTimeoutMs = 0 });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Succeeded);
+    }
+
+    [Fact]
+    public void Validate_Succeeds_WhenWorkerReadyWaitTimeoutMsIsPositive()
+    {
+        GatewayOptions options = CloneWithSessions(
+            ValidOptions(),
+            new SessionOptions { WorkerReadyWaitTimeoutMs = 5000 });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Succeeded);
+    }
 }