Found while load-verifying #856: ShutdownAsync_cancels_the_event_stream_and_releases_the_pump
timed out 7/700 under parallel load. The fake's StreamCancelled was set by a
CancellationToken registration scoped by `using` inside the async iterator.
On Cancel() the newer ReadAllAsync registration runs first and resumes the
iterator on a pool thread, whose `using` can dispose our registration while
Cancel() is still walking the callback list - so it never runs. Signal from
the iterator's finally (when the token is cancelled) instead, which also
proves the stream actually ended. 0/700 after.
Claude-Session: https://claude.ai/code/session_01GT67AD99zQQqpLCVm3p7xk
Joseph Doherty
90de38910d