The child created the drained file before writing it, so the test's
exists() poll could read it empty (CI run 37875702339: left: []).
Write to a temp file and rename.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>