Fetches one page for a given body. Decoupled from the facade's ec arg.
isFirstPage is true only for the initial fetch of this iteration run
(each pages()/items()/toArray() call starts a fresh run). Adapters that
carry per-first-page state (e.g. an eventual-consistency window) must key off
this flag rather than closure state so that reusing a paginator, or consuming
two views concurrently, does not leak first-page behaviour across iterators.
Fetches one page for a given body. Decoupled from the facade's
ecarg.isFirstPageistrueonly for the initial fetch of this iteration run (eachpages()/items()/toArray()call starts a fresh run). Adapters that carry per-first-page state (e.g. an eventual-consistency window) must key off this flag rather than closure state so that reusing a paginator, or consuming two views concurrently, does not leak first-page behaviour across iterators.