Terminate timed-out Git process trees (#36793)

## 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

codex-rs/git-utils/src/lib.rs

@@ -3,6 +3,7 @@ mod baseline;
 mod branch;
 mod errors;
 mod fsmonitor;
+mod git_process;
 mod info;
 mod operations;
 mod platform;