Adds contacts to an existing campaign. New contacts are queued as SCHEDULED. Behaviour depends on the campaign status: for an active campaign (SCHEDULED or ON_GOING), if today's daily window is still open and maxContactsPerDay has remaining quota, a send catch-up starts immediately with a batch prorated to the remaining window and capped by the unused daily quota; if the window is closed or the daily quota is exhausted, contacts wait for the next daily start. A completed campaign is restarted via the normal schedule path. A paused, draft, or pending-approval campaign only queues the contacts (they are sent when the campaign is activated or resumed). Phone numbers already present in the campaign are skipped. At least one of 'selection', 'contacts' or 'phoneNumbers' must be provided.