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