addContactsToList adds contacts identified by email and reports provider
successes and failures separately.
Example
list: string | number and a non-empty emails array are required. Contacts
must already exist; this action does not upsert them. account selects the
Brevo binding.
Returns { success: string[]; failure: string[] }. Partial failure is a normal
provider result rather than an action error, so inspect failure before
assuming every membership changed.