Add contacts to a group (v2)

Add one or more contacts to a group by providing an array of contact IDs. Contacts that are already members are silently skipped server-side. Requires Idempotency-Key. Returns { data: { affected, ids, remaining? } } where affected is the count of NEW memberships created, ids echoes the input contact IDs that were applied. Send allowPartial: true to accept a partial success: remaining then lists input IDs the server could not apply inside its request budget — nothing about them was committed, so resubmit exactly those under a new Idempotency-Key. Without allowPartial, a write that cannot finish returns a retryable 500 timeout; retrying the same list is safe.