Merge duplicate contacts (v2)

Merge one or more groups of duplicate contacts. The request body is { contactIds: Group[] } where each group is either a plain array of ≥2 contact IDs (the OLDEST record survives) or { contact_ids: string[], primary_contact_id?: string } to choose which record survives. Returns { data: { merged: Contact[], failed: [...] } }: merged holds the surviving contacts, failed lists any groups that could not be merged — each with error.code, e.g. merge_contact_not_found when a group names a contact that does not exist or is not yours (the group is reported, never silently skipped). Archived contacts can be merged; the survivor is active when any member of its group was active and stays archived only when every member was archived. Idempotency-Key is REQUIRED.