Kill process group on flow run cancellation

[wavebyrd]

Summary

Fixes #20979.

When a flow run is cancelled, the worker sends SIGTERM only to the flow run process itself. Any child processes (like those from ShellOperation) are left orphaned.

This PR:

• Starts flow run subprocesses in their own process group (start_new_session=True on POSIX)
• Switches cancellation signals from os.kill to os.killpg, targeting the whole process group

This covers all three cancellation/kill paths: Runner._kill_process, ProcessManager.kill, and Runner.reschedule_current_flow_runs. The new EngineCommandStarter path also gets start_new_session=True.

Windows behavior is unchanged (still uses CTRL_BREAK_EVENT).

Test plan

• Updated existing test__process_manager.py tests to verify killpg/getpgid usage
• Manual: deploy flow with ShellOperation(commands=["sleep 9999"]) to a process work pool, cancel via UI, confirm sleep process is terminated

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing wavebyrd:fix/kill-process-group-on-cancel (2ec67b1) with main (447059a)}

[desertaxle]

Thanks for opening a PR @wavebyrd!

Please run ruff on your changes so that the static analysis stage in our checks passes. Also, it looks like these changes are causing the ' pytest ' process to be killed during the test run. That will need to be fixed before we can approve and merge this PR.