Add base64 error message

[nshiro]

base64 のエラーメッセージを追加しました🍹
（本体のリリースはまだですが）

元はこちらです。
https://github.com/laravel/framework/pull/60808/changes

[what-the-diff]

PR Summary

• Added a New Validation Message
  A new warning message has been included in the system. This will display when users input an invalid Base64 string, helping them to rectify the error quickly. This update is particularly important for users interacting with the platform in Japanese, as the validation message is added to the Japanese language file specifically.

[askdkc]

@nshiro さん

ありがとうございます〜
本体側でマージ済みなので、こっちもマージですね👍