## Why
Timing out a Git metadata command must not leave helper processes running after
the command wrapper exits.
## What changed
- Run Git metadata commands in a dedicated process group on Unix and a Job
Object on Windows so timeout cleanup terminates their full process trees.
- Start Windows commands suspended, assign them to the Job Object, and then
resume them so immediate descendants cannot escape containment.
- Preserve descendants when a Git command completes normally, and retain the
existing direct-spawn fallback if Windows Job Object setup fails.
## Testing
Added cross-platform regression tests for cleanup both while the command wrapper
is running and after it exits, plus Windows coverage for immediate-child Job
Object containment.
GitOrigin-RevId: 351851708e23ff06b89fe1894bd09a3558f67293
