fix: keep stderr written before an async timeout

[jqnatividad]

Problem

On timeout, AsyncQsv raised QsvTimeout with an empty or truncated stderr, while Qsv kept everything qsv wrote before the deadline.

run_async wrapped proc.communicate() in asyncio.wait_for. On timeout, wait_for cancels it, and the inner StreamReader.read(-1) collects chunks in a local list, so whatever it had already read was thrown away. _timeout_cleanup then only saw stderr arriving after the kill.

Fix

run_async now waits on the communicate() task with asyncio.wait(..., timeout=) and doesn't cancel it on timeout. The process tree is killed as before, then the still-running read is given kill_grace to finish and its output is used. If a descendant that left the process group still holds the pipes, the read is cancelled and the output dropped, the same as run_sync. Cancellation and teardown paths are otherwise unchanged.

Tests

• New test_timeout_keeps_stderr_written_before_it (sync) and test_async_timeout_keeps_stderr_written_before_it. The async one failed before the fix.
• Full suite passes locally (63 passed, macOS). ruff and mypy are clean (both default and --platform win32).
• The async tests produce no asyncio "Task was destroyed" or "never retrieved" log messages.
• The change is platform-neutral but has not been run on Windows locally; CI covers it.

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