Process-pool hang via undrained standard-error pipe

Denial of service in AnyIO before 4.14.2 allows local users to hang process-pool calls by flooding worker standard error. The process_worker path leaves sys.stderr connected to a parent pipe that is never drained, so a full buffer blocks the worker before its protocol reply. Exploitation requires code run through to_process.run_sync() to emit substantial stderr data; the resulting call remains blocked indefinitely.