Closes ZED-86Q

Fixes this (rare) crash by making entity creation via a window context
infallible.

The shape of the result is a bit clunky due to the type signatures
involved. The real solution is to remove AsyncWindowContext entirely and
move it's fallible APIs into WindowHandle, but until then this will give
you a simple starting point.

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 is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A