● Closes #50285

## Summary

When `inline_blame` is disabled in settings and `editor::BlameHover` is
triggered,
the blame popover fails to appear on the first invocation because
`start_git_blame`
kicks off an async task to fetch blame data, but `blame_hover`
immediately tries to
read that data synchronously before it's available.

This fix registers a one-shot observation on the blame entity when it's
freshly
created, so the popover is shown once blame data has finished
generating.

Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [ ] ~Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)~
(No UI changes)

## Videos:
### Before:

https://github.com/user-attachments/assets/d2ca4f8e-8186-49af-9908-5a82bfca0de2


Ps: pressed the keybind two times

### After:

https://github.com/user-attachments/assets/08575cd9-2bbc-462d-92fa-f0e7ef23d8d6



## Release Notes:

- Fixed blame hover popover not appearing on first trigger when inline
blame is disabled (#50285)

---------

Co-authored-by: Chris Biscardi <chris@christopherbiscardi.com>