Closes#45211
This ensures that all sub-processes that were launched by the ACP server
are terminated. One scenario where this is easily reproducible:
- Start a new Claude Code ACP session
- Submit a prompt
- While Claude-code is still responding, start a new session
- The `claude-code` subprocess is leaked from the previous session (The
Claude-code SDK runs the Claude-code binary in a sub process)
This PR fixes this by using process groups on Unix.
It does not fix the process leaks on Windows yet (will follow up with
another PR)
Release Notes:
- Fixed an issue where subprocesses of ACP servers could be leaked after
starting a new session
9f599466b5