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/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)