Get param class type in NestJS cutom param decorator

Question

I have a NestJS TS application that has an xml endpoint. I want to validate an xml body. Here's how I went with parsing xml to js object:

@Injectable()
export class XmlToJsInterceptor implements NestInterceptor {
  constructor(private parser: CxmlParserService) {}

  public intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
    const req: XmlRequest<unknown> = context.switchToHttp().getRequest();
    if (req.rawBody) {
      req.xml = this.parser.convertXMLToObject(req.rawBody) as unknown;
    }
    return next.handle();
  }
}

export const XmlBody = createParamDecorator((data: unknown, ctx: ExecutionContext): unknown => {
  const request: XmlRequest<unknown> = ctx.switchToHttp().getRequest();
  return request.xml;
});

And I use it like this:

@UseInterceptors(XmlToJsInterceptor)
@Controller('/endpoint')
export class MyController {

  @Post('/')
  @HttpCode(HttpStatus.OK)
  async handleEndpoint(
    @XmlBody() body: MyClassValidator,
  ): Promise<void> {

Now I want to use class-validator to check if xml request has a proper structure. I thought to extend XmlBody nestjs param decorator to include validation and manually call class-validator like this:

export const XmlBody = createParamDecorator((data: unknown, ctx: ExecutionContext): unknown => {
  const request: XmlRequest<unknown> = ctx.switchToHttp().getRequest();

  const validatedConfig = plainToClass(HOWDO_I_GET_PARAM_CLASS, request.xml, {
    enableImplicitConversion: true,
  });
  const errors = validateSync(validatedConfig, { skipMissingProperties: false });

  if (errors.length > 0) {
    throw new Error(errors.toString());
  }
  return request.xml;
});

But I don't know to get typescript class from annotated parameter.