Support full-portfolio tokens & improve WABA auth error

This MR fixes a false-negative rejection in WhatsAppValidatorService when validating Meta access tokens that have been granted full portfolio access (i.e. the token holds a whatsapp_business_messaging or whatsapp_business_management scope, but the target_ids array is absent or empty).

It also improves the error message surfaced when a Business Account ID is not authorized by the token, replacing a vague message with a detailed, actionable one.