/// Trait for types that can sanitize their own fields.
///
/// Implement this on request/input structs to normalize data
/// (trim whitespace, lowercase emails, etc.) before validation.
///
/// The `JsonRequest`, `FormRequest`, `Query`, and `MultipartRequest` extractors
/// call [`Sanitize::sanitize`] automatically after deserialization, so every bound on
/// those extractors requires `T: Sanitize`.