Add Korean (ko) localization for Volo.Abp.Validation module

[moduvoice]

Motivation

This adds Korean (ko) translation support — to help Korean-speaking users use this
open-source project more comfortably in their own language.
(한국 사람들의 오픈소스 이용에 도움이 되게 하기 위해서 한글화 작업을 하였습니다.)

ABP is a very large framework with hundreds of modules, so this PR starts with the
Volo.Abp.Validation module (one of the smallest open-source core packages) as a first
step; additional modules will follow in separate PRs.
(abp는 모듈이 수백 개인 대형 프레임워크이므로, 이번 PR에서는 오픈소스 코어 패키지 중 가장 작은 축에
속하는 Volo.Abp.Validation 모듈부터 시작합니다. 이후 다른 모듈들은 별도의 PR로 추가할 예정입니다.)

Changes

• Added framework/src/Volo.Abp.Validation/Volo/Abp/Validation/Localization/ko.json
• No additional registration was needed: AbpValidationModule already loads all
  localization files in this folder via AddVirtualJson("/Volo/Abp/Validation/Localization"),
  so the new ko.json file is picked up automatically.

Testing

• Verified that ko.json contains exactly the same set and order of keys as en.json
  (34/34 keys, no missing/extra/empty values) using a Python script comparing both
  JSON files' texts objects.
• Verified the JSON is well-formed (parsed successfully with json.load).
• Followed the "Manual Translation" approach described in
  docs/en/contribution/index.md,
  which does not require a pre-registered issue for open-source core modules
  (only commercial modules require an issue first).