refactor(battle-node): SocketIoFrame throws on namespace; typed JSON construction

SVSim.UnitTests/BattleNode/Wire/SocketIoFrameTests.cs

@@ -105,6 +105,16 @@ public class SocketIoFrameTests
         Assert.That(text, Is.EqualTo(wire));
     }
 
+    [Test]
+    public void Parse_NamespacePrefix_Throws()
+    {
+        // v1 only supports the default namespace. A "/foo," prefix used to be silently
+        // skipped, which would route a frame meant for namespace /foo to the default
+        // handler. Fail loud instead so we'd notice if the client ever started using one.
+        var ex = Assert.Throws<ArgumentException>(() => SocketIoFrame.Parse("2/foo,[\"msg\"]"));
+        Assert.That(ex!.Message, Does.Contain("/foo"));
+    }
+
     [Test]
     public void Encode_EventNameWithSpecialChars_IsJsonEscaped()
     {