One page of results from a list endpoint.
data holds the items. The API has two cursor styles and this struct
carries both, so the fields the endpoint doesn't use are nil:
- Models and Message Batches use id cursors — go forward with
after_id: page.last_id, back withbefore_id: page.first_id, and checkhas_moreto know whether there is a next page. - Files uses an opaque cursor — pass
page: page.next_pageto get the next page, and stop whennext_pageis nil.