A task that floods stdout could not be stopped promptly: cancelling it
(Ctrl+C from the thin client) stopped the task, but the server kept
draining the already-queued output to the client for several seconds,
because onCancellationRequest never discarded the pending backlog.
onCancellationRequest now sets a per-channel isCanceled flag and clears the
queued frames and buffered stdout (discardPending), and jsonRpcNotify
drops the cancelled task's systemOut/systemErr while the flag is set, so
output stops promptly.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
BrianHotopp