feat(go): add single-shot Write2 session helper (§4.1)

Add Write2/Write2Raw to the Go client Session, mirroring the existing
Write/WriteRaw pair, so all five language clients now expose write2.
Includes three TDD tests covering payload propagation, raw-reply return,
and nil-value rejection.

clients/go/mxgateway/session.go

@@ -580,6 +580,35 @@ func (s *Session) WriteRaw(ctx context.Context, serverHandle, itemHandle int32,
 	})
 }
 
+// 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)
+	return err
+}
+
+// Write2Raw invokes MXAccess Write2 (timestamped single-item write) and returns the raw reply.
+func (s *Session) Write2Raw(ctx context.Context, serverHandle, itemHandle int32, value, timestampValue *MxValue, userID int32) (*MxCommandReply, error) {
+	if value == nil {
+		return nil, errors.New("mxgateway: write2 value is required")
+	}
+	if timestampValue == nil {
+		return nil, errors.New("mxgateway: write2 timestamp value is required")
+	}
+
+	return s.invokeCommand(ctx, &pb.MxCommand{
+		Kind: pb.MxCommandKind_MX_COMMAND_KIND_WRITE2,
+		Payload: &pb.MxCommand_Write2{
+			Write2: &pb.Write2Command{
+				ServerHandle:   serverHandle,
+				ItemHandle:     itemHandle,
+				Value:          value,
+				TimestampValue: timestampValue,
+				UserId:         userID,
+			},
+		},
+	})
+}
+
 // Events streams ordered session events until the server ends the stream,
 // context cancellation stops Recv, or a terminal error is sent.
 func (s *Session) Events(ctx context.Context) (<-chan EventResult, error) {