fix: add timeout to _shutting_down_fut await in SupervisedProc.aclose()

[svacatalisan]

Without a timeout, aclose() hangs indefinitely if the child process acks shutdown but never sends ShuttingDown (e.g. session.aclose() gets stuck). This wraps the await with wait_for(close_timeout) and kills the process on timeout, making the shutdown sequence fully bounded.

[CLAassistant]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.