test(worker): use Register (a real STA command) for STA-dispatch race tests

Ping is now intercepted as a worker control command and answered on the
message-loop thread, so the dispatch/heartbeat/shutdown-race tests must use a
genuine STA-dispatched command kind to keep exercising DispatchAsync.

src/ZB.MOM.WW.MxGateway.Worker.Tests/Ipc/WorkerPipeSessionTests.cs

@@ -1130,6 +1130,11 @@ public sealed class WorkerPipeSessionTests
         };
     }
 
+    // A generic STA-dispatched command used by the dispatch/heartbeat/
+    // shutdown-race tests. Register is a real MXAccess command kind (not a
+    // worker control command), so it flows through IWorkerRuntimeSession
+    // .DispatchAsync — unlike Ping/GetSessionState/etc., which are answered on
+    // the message-loop thread without touching the STA.
     private static WorkerEnvelope CreateCommandEnvelope(string correlationId, ulong sequence = 2)
     {
         return new WorkerEnvelope
@@ -1142,10 +1147,10 @@ public sealed class WorkerPipeSessionTests
             {
                 Command = new MxCommand
                 {
-                    Kind = MxCommandKind.Ping,
+                    Kind = MxCommandKind.Register,
-                    Ping = new PingCommand
+                    Register = new RegisterCommand
                     {
-                        Message = "ping",
+                        ClientName = "test-client",
                     },
                 },
                 EnqueueTimestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),