## Why
The native macOS backend cannot inspect every setting or callback on a Tokio command. A shared, constrained launch API makes the supported settings explicit for both backends.
## What changed
- Replace `spawn_child` with `codex_utils_pty::Command`, enforcing an explicitly supplied environment, piped stdio, and kill-on-drop behavior.
- Add `ProcessMode::Inherit` and `ProcessMode::NewGroup`, honoring the selected mode in native macOS and Tokio spawning.
- Share the `Child` wrapper across platforms and migrate local MCP server launching to the new API, retaining its new process group and Windows suspended-spawn support.
## Testing
Adapt the existing macOS spawn compatibility and child lifecycle tests to the new API. Add a regression test that checks inherited and new process groups with both backends.
GitOrigin-RevId: 3e7a401659e3a8a94a13c4ff58d67bdcd2eea33d
3801fc8dea