fix(code-run): terminate descendant processes

[Whxuan0701]

Closes #797

What

• Starts each POSIX code_run in its own session and terminates the process group on stop/timeout.
• Tracks Windows descendants with a Job Object, with taskkill /T /F as a fallback when job assignment is unavailable.
• Uses a bounded graceful-then-forceful stop on POSIX and retains the existing result/status contract.
• Adds regression tests in which a delayed child side effect must not occur after manual stop or timeout.

Why

The previous process.kill() only terminated the direct child. A command could report [Stopped] or [Timeout Error] while grandchildren continued running, retained resources, or modified files later.

Validation

• Parent baseline f6e5657: delayed child canary still appears after timeout.
• macOS 14.6.1: frontends/tests/test_code_run_process_tree.py — 2 passed.
• Python 3.10.20, 3.12.13, and 3.13.14 focused tests passed.
• Full non-release frontend suite: 242 passed.
• Release qualification suite: 35 passed.
• python -m py_compile ga.py and git diff --check passed.

Boundary

The POSIX behavior was exercised on macOS. The Windows Job Object path was syntax-checked and is covered structurally by the same implementation, but I did not have a Windows host for an end-to-end descendant canary run; Windows CI feedback is welcome while this remains a draft.

Review note

The Windows Popen-to-Job assignment has an unavoidable short creation window with the stdlib API. A child that forks and exits before assignment can escape the job. If maintainers need a strict Windows guarantee, I suggest following up with suspended process creation plus assignment before resume; I kept that lower-level change out of this focused patch.

[Whxuan0701]

Implementation and regression tests are ready in this draft. The POSIX process-group path was exercised on macOS 14.6.1; the Windows Job Object path is included but awaits Windows CI/maintainer validation.