The two cancellation tests each built their own `slow_task`, so the
documented showcase — the config file, its preset registration, and the
fixture's own abort loop — was covered by nothing. A misspelt preset name
or a regression in that handler would have surfaced only when someone ran
the repro by hand.
Resolve `cancellation-modern-http.json` through `loadConfig`/`resolveConfig`
and drive the real preset, following the pattern the nullable-fields and
MRTR showcase tests already use.
It asserts on the handler's own return value rather than on progress
notifications. Counting ticks is a false negative here: the SDK drops a
cancelled request's progress handler locally, so the client stops seeing
ticks the moment it cancels whether or not the server ever stopped —
verified, that version passed with the fix reverted.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013hzwzS8UvBsr4yZm7o7sev
Signed-off-by: cliffhall <cliff@futurescale.com>