Remove contacts from a group (v2)

Remove one or more contacts from a group. The contacts themselves are NOT deleted — only their groups_contacts membership rows are removed. Requires Idempotency-Key. Returns { data: { affected, ids, remaining? } } where affected is the count of rows actually deleted, 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 — 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.