Aborts the run of a session (or of each session in a list): transfers are canceled, child
processes stopped, the partial turn recorded as aborted, queued items moved to dropped, and
the status set to aborted. Idle sessions are left alone.
Examples
session = peter("long task", model = gptr_fake_provider(list(list(hang = TRUE))),
.run = FALSE)
gptr_cancel(session)