The ReadableStream cancel() callback is not reliably invoked on client
disconnect under Next.js, causing emitter listeners (line/clear) and the
keepalive interval to accumulate, eventually triggering
MaxListenersExceededWarning.
Use request.signal as the primary disconnect trigger, with cancel() and
enqueue failures as fallbacks. Cleanup is idempotent via state.closed.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
