Allow `uv format` in unmanaged projects

e.g., from the uv repository

```
❯ echo "x   = 1" > foo.py
❯ uv format -- foo.py
warning: `uv format` is experimental and may change without warning. Pass `--preview-features format` to disable this warning.
error: The project is marked as unmanaged: `/Users/zb/workspace/.worktrees/uv/zb-lock-bin-install`
```

We should definitely allow this. I presume it regressed in #15438 or #15440 