Closes https://github.com/zed-industries/zed/issues/34991

Before, the picker did not allow to open the current directory that was
just completed:

<img width="553" height="354" alt="image"
src="https://github.com/user-attachments/assets/e77793c8-763e-416f-9728-18d5a39b467f"
/>

pressing `enter` here would open `assets`; pressing `tab` would append
the `assets/` segment to the query.
Only backspace, removing `/` would allow to open the current directory.

After:
<img width="574" height="349" alt="image"
src="https://github.com/user-attachments/assets/bdbb3e23-7c7a-4e12-8092-51a6a0ea9f87"
/>

The first item is now a placeholder for opening the current directory
with `enter`.
Any time a fuzzy query is appended, the placeholder goes away; `tab`
selects the entry below the placeholder.

Release Notes:

- Made fallback open picker more intuitive

---------

Co-authored-by: Peter Tripp <petertripp@gmail.com>
Co-authored-by: David Kleingeld <davidsk@zed.dev>