fix: The @IsByteLength() decorator does not validate correctly for multi-byte characters such as Japanese and Chinese.

[Suyama-Daichi]

Description

I am developing a service that targets Japanese.
I used the IsByteLength() decorator to limit the message length of the request body to 2048 bytes.
However, it is not being validated as expected.

Minimal code-snippet showcasing the problem

I created a reproducible repository.
https://github.com/Suyama-Daichi/class-validator-2byte

The following code is an excerpt.

import { ApiProperty } from '@nestjs/swagger';
import { IsByteLength, IsNotEmpty, IsString } from 'class-validator';

export class RequestDTO {
  @ApiProperty({
    example: 'テスト',
    description: 'message (Japanese)',
    type: String,
  })
  @IsNotEmpty()
  @IsString()
  @IsByteLength(0, 6)
  message: string;
}

import { Body, Controller, Post } from '@nestjs/common';
import { RequestDTO } from './RequestDTO';
import { ApiOperation } from '@nestjs/swagger';

@Controller()
export class AppController {
  constructor() {}

  @ApiOperation({ summary: 'Post name' })
  @Post()
  postName(@Body() body: RequestDTO) {
    return body;
  }
}

Expected behavior

If you send the following request:

curl -X 'POST' \
  'http://localhost:3000/' \
  -H 'accept: */*' \
  -H 'Content-Type: application/json' \
  -d '{
  "message": "テスト"
}'

The validation should pass and you should get the following response:

{
  "message": "テスト"
}

Actual behavior

If you send the following request:

curl -X 'POST' \
  'http://localhost:3000/' \
  -H 'accept: */*' \
  -H 'Content-Type: application/json' \
  -d '{
  "message": "テスト"
}'

A validation error will occur.
テスト should be 6 bytes, so it should pass validation.

{
  "message": [
    "message's byte length must fall into (0, 6) range"
  ],
  "error": "Bad Request",
  "statusCode": 400
}

Related issue

#775