feat(go): add ping CLI subcommand (§4.3)

Add PingRaw to Session (session.go), runPing to the CLI dispatch
(main.go), and three tests covering plain-text echo, JSON output,
and missing-session-id validation (main_test.go). Default message
is "ping"; gateway echo is read from DiagnosticMessage, falling
back to the kind string if absent.

clients/go/cmd/mxgw-go/main.go

@@ -121,6 +121,8 @@ func runWithIO(ctx context.Context, args []string, stdout, stderr io.Writer) err
 		return runGalaxyDiscover(ctx, args[1:], stdout, stderr)
 	case "galaxy-watch":
 		return runGalaxyWatch(ctx, args[1:], stdout, stderr)
+	case "ping":
+		return runPing(ctx, args[1:], stdout, stderr)
 	case "batch":
 		return runBatch(ctx, os.Stdin, stdout, stderr)
 	default:
@@ -228,6 +230,47 @@ func runCloseSession(ctx context.Context, args []string, stdout, stderr io.Write
 	return nil
 }
 
+func runPing(ctx context.Context, args []string, stdout, stderr io.Writer) error {
+	flags := flag.NewFlagSet("ping", flag.ContinueOnError)
+	flags.SetOutput(stderr)
+	common := bindCommonFlags(flags)
+	jsonOutput := flags.Bool("json", false, "write JSON output")
+	sessionID := flags.String("session-id", "", "gateway session id")
+	message := flags.String("message", "ping", "ping payload message")
+
+	if err := flags.Parse(args); err != nil {
+		return err
+	}
+	if *sessionID == "" {
+		return errors.New("session-id is required")
+	}
+
+	client, options, err := dialForCommand(ctx, common)
+	if err != nil {
+		return err
+	}
+	defer client.Close()
+
+	session := mxgateway.NewSessionForID(client, *sessionID)
+	reply, err := session.PingRaw(ctx, *message)
+	if err != nil {
+		return err
+	}
+	if *jsonOutput {
+		return writeJSON(stdout, commandReplyOutput{
+			Command: "ping",
+			Options: options,
+			Reply:   mustMarshalProto(reply),
+		})
+	}
+	echo := reply.GetDiagnosticMessage()
+	if echo == "" {
+		echo = reply.GetKind().String()
+	}
+	fmt.Fprintln(stdout, echo)
+	return nil
+}
+
 func runRegister(ctx context.Context, args []string, stdout, stderr io.Writer) error {
 	flags := flag.NewFlagSet("register", flag.ContinueOnError)
 	flags.SetOutput(stderr)
@@ -1196,7 +1239,7 @@ type protojsonMessage interface {
 }
 
 func writeUsage(writer io.Writer) {
-	fmt.Fprintln(writer, "usage: mxgw-go <version|open-session|close-session|register|add-item|advise|subscribe-bulk|unsubscribe-bulk|read-bulk|write-bulk|write2-bulk|write-secured-bulk|write-secured2-bulk|bench-read-bulk|write|stream-events|stream-alarms|acknowledge-alarm|smoke|galaxy-test-connection|galaxy-last-deploy|galaxy-discover|galaxy-watch|batch>")
+	fmt.Fprintln(writer, "usage: mxgw-go <version|open-session|close-session|ping|register|add-item|advise|subscribe-bulk|unsubscribe-bulk|read-bulk|write-bulk|write2-bulk|write-secured-bulk|write-secured2-bulk|bench-read-bulk|write|stream-events|stream-alarms|acknowledge-alarm|smoke|galaxy-test-connection|galaxy-last-deploy|galaxy-discover|galaxy-watch|batch>")
 }
 
 // batchEOR is the end-of-result sentinel emitted to stdout after every command

clients/go/cmd/mxgw-go/main_test.go

@@ -190,6 +190,102 @@ func TestRunBenchReadBulkRespectsContextCancellation(t *testing.T) {
 	}
 }
 
+// TestRunPingPlainText verifies the ping subcommand round-trips through the
+// fake gateway and prints the echo (diagnostic_message) in plain-text mode.
+func TestRunPingPlainText(t *testing.T) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	server := grpc.NewServer()
+	fake := &pingFakeGateway{}
+	pb.RegisterMxAccessGatewayServer(server, fake)
+	go func() { _ = server.Serve(listener) }()
+	defer server.Stop()
+	defer listener.Close()
+
+	var stdout, stderr bytes.Buffer
+	args := []string{
+		"ping",
+		"-endpoint", listener.Addr().String(),
+		"-plaintext",
+		"-api-key", "test",
+		"-session-id", "test-session",
+		"-message", "hello",
+	}
+	if err := runWithIO(t.Context(), args, &stdout, &stderr); err != nil {
+		t.Fatalf("runWithIO() error = %v; stderr = %s", err, stderr.String())
+	}
+	got := strings.TrimSpace(stdout.String())
+	if got != "pong:hello" {
+		t.Fatalf("ping plain-text output = %q, want %q", got, "pong:hello")
+	}
+}
+
+// TestRunPingJSON verifies the ping subcommand emits valid JSON in --json mode.
+func TestRunPingJSON(t *testing.T) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	server := grpc.NewServer()
+	fake := &pingFakeGateway{}
+	pb.RegisterMxAccessGatewayServer(server, fake)
+	go func() { _ = server.Serve(listener) }()
+	defer server.Stop()
+	defer listener.Close()
+
+	var stdout, stderr bytes.Buffer
+	args := []string{
+		"ping",
+		"-endpoint", listener.Addr().String(),
+		"-plaintext",
+		"-api-key", "test",
+		"-session-id", "test-session",
+		"-message", "hello",
+		"-json",
+	}
+	if err := runWithIO(t.Context(), args, &stdout, &stderr); err != nil {
+		t.Fatalf("runWithIO() error = %v; stderr = %s", err, stderr.String())
+	}
+	var out commandReplyOutput
+	if err := json.Unmarshal(stdout.Bytes(), &out); err != nil {
+		t.Fatalf("parse JSON: %v\noutput: %s", err, stdout.String())
+	}
+	if out.Command != "ping" {
+		t.Fatalf("command = %q, want %q", out.Command, "ping")
+	}
+}
+
+// TestRunPingRequiresSessionID verifies the ping subcommand rejects missing session-id.
+func TestRunPingRequiresSessionID(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	err := runWithIO(t.Context(), []string{"ping", "-plaintext", "-api-key", "test"}, &stdout, &stderr)
+	if err == nil {
+		t.Fatalf("runWithIO(ping without --session-id) returned no error")
+	}
+	if !strings.Contains(err.Error(), "session-id is required") {
+		t.Fatalf("error = %v; want 'session-id is required'", err)
+	}
+}
+
+// pingFakeGateway handles Invoke for MX_COMMAND_KIND_PING by echoing the
+// message back in the diagnostic_message field so the CLI plain-text path
+// has a deterministic, non-empty string to assert on.
+type pingFakeGateway struct {
+	pb.UnimplementedMxAccessGatewayServer
+}
+
+func (g *pingFakeGateway) Invoke(_ context.Context, req *pb.MxCommandRequest) (*pb.MxCommandReply, error) {
+	echo := "pong:" + req.GetCommand().GetPing().GetMessage()
+	return &pb.MxCommandReply{
+		SessionId:         req.GetSessionId(),
+		Kind:              pb.MxCommandKind_MX_COMMAND_KIND_PING,
+		DiagnosticMessage: echo,
+		ProtocolStatus:    &pb.ProtocolStatus{Code: pb.ProtocolStatusCode_PROTOCOL_STATUS_CODE_OK},
+	}, nil
+}
+
 // benchFakeGateway is a minimal MxAccessGatewayServer that satisfies the
 // bench-read-bulk session-setup sequence (OpenSession + Invoke for Register
 // / SubscribeBulk / ReadBulk / UnsubscribeBulk / CloseSession).

clients/go/mxgateway/session.go

@@ -580,6 +580,17 @@ func (s *Session) WriteRaw(ctx context.Context, serverHandle, itemHandle int32,
 	})
 }
 
+// PingRaw sends a diagnostic PING command and returns the raw reply.
+// The message is echoed back by the gateway in the reply's DiagnosticMessage field.
+func (s *Session) PingRaw(ctx context.Context, message string) (*MxCommandReply, error) {
+	return s.invokeCommand(ctx, &pb.MxCommand{
+		Kind: pb.MxCommandKind_MX_COMMAND_KIND_PING,
+		Payload: &pb.MxCommand_Ping{
+			Ping: &pb.PingCommand{Message: message},
+		},
+	})
+}
+
 // Write2 invokes MXAccess Write2 (timestamped single-item write).
 func (s *Session) Write2(ctx context.Context, serverHandle, itemHandle int32, value, timestampValue *MxValue, userID int32) error {
 	_, err := s.Write2Raw(ctx, serverHandle, itemHandle, value, timestampValue, userID)