The echo-agent fixture's prompt handler awaits a sessionUpdate write before
registering the cancellable hang; the cancel notification is dispatched
concurrently and could land first on loaded CI shards, no-op, and leave the
prompt hanging forever (5s test timeout on shard 2). The fixture now records a
pending cancel so prompt() resolves 'cancelled' immediately regardless of
arrival order. Test-fixture-only change; verified 5x locally.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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