OptionalsignalAborts the request, and any retry back-off it is waiting out.
The reason the signal carries is rethrown untouched — a DOMException named AbortError by default, whatever was
passed to abort() otherwise, and a TimeoutError from AbortSignal.timeout(). It is never wrapped in a
NetworkError: an abort is not a transport failure, and callers branch on the reason they supplied.
Per-call transport options — the last argument of every generated operation.
Kept apart from the operation's own parameters, which are a faithful projection of the specification: a field named here can never be mistaken for one the API declares, and the exported parameter types stay exactly what they describe.