ShouldReturnFromSendSyncQUIC: 500 rounds of four fresh connections, each
sending one message on its first stream to a server that echoes it with
a FIN and drops the stream; every SendSync must return and every echo
come back. The race it guards needs the sending thread descheduled for
a round trip, so it is a stress test, not a deterministic one: against
the previous SendSync it failed 6 runs of 7, with one or two of the 2000
sends throwing each time.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
