## Why
Windows filesystem operations can complete while their helper process remains
alive, and elevated sandbox processes must not survive a disconnected control
pipe.
## What changed
- Read newline-delimited filesystem helper responses without imposing an
operation deadline, then bound the time spent reaping the helper.
- Drain helper stderr concurrently, retain a bounded diagnostic on failure, and
kill helpers that do not exit after responding.
- Terminate the elevated runner's job or process when its input pipe closes or
fails.
## Testing
Added coverage for delayed responses, noisy stderr, stuck helpers, duplicated
file handles, and termination of elevated descendants after a control-pipe
disconnect.
GitOrigin-RevId: c0d3d5d76e13318de17638df4db71fcfb9a7397f
21c58c90f2