Make File size validation messages human-readable

Yiisoft\Validator\Rule\File size validation placeholders currently expose raw byte counts.

Example:

new File(
    maxSize: 50 * 1024 * 1024,
    tooBigMessage: "{file} is larger than {limit}.",
)

{limit} resolves to 52428800, which is not convenient for user-facing upload validation messages.

Preferred behavior: use the same existing placeholders, but format size values in a human-readable way:

• {limit} -> 50 MB
• {exactly} -> 50 MB

This would let applications use Yii Validator defaults/placeholders without duplicating byte-to-readable-size formatting in application code.