Wrap the fds returned by `create_pipe` and `open_dev_null` in
`std::fs::File` earlier, so that they are closed if we return early due
to an error. Includes a regression test.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed a file descriptor leak on macOS.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>