SmsLinkItem missing link validation constraints

Problem/Motivation

SmsLinkItem is missing all link validation constraints. PHP attributes don't inherit from parent to child class, so SmsLinkItem's #[FieldType] attribute (not re-specifying constraints) silently dropped LinkType/LinkAccess/LinkExternalProtocols/LinkNotExistingInternal — meaning sms_link fields accepted any URI unvalidated, including javascript:.

Proposed resolution

Explicitly re-declare the same constraints core's LinkItem uses.