Set custom-field values on contacts (v2)

Batch-set per-contact custom-field VALUES (the contacts_custom_fields rows, not the field definitions). Body: { custom_fields: [{ contact_id, custom_field_id, text_value?, array_value?, date_value?, mode?, separator? }] }. mode is replace (default, overwrites), prepend or append — the latter two keep the stored value and add the new one before/after it atomically on the server (text joined by separator, default newline; arrays concatenated and de-duplicated; date_value only supports replace). Requires Idempotency-Key. Returns { data: { affected } } (count of successful updates; per-row failures are skipped and logged).