Type Alias TranslatorFn 

pub type TranslatorFn = fn(&str, &[(&str, &str)]) -> Option<String>;

Callback signature for validation message translation.

• key: Translation key (e.g., "validation.required")
• params: Interpolation parameters (e.g., [("attribute", "email"), ("min", "8")])
• Returns: Translated message, or None if the key is not found (caller falls back to English)