assistant2: Combine file & directory picker (#26975)

In the process of adding `@mentions` we realized that we do not want to
make a distinction between Files & Directories in the UI, therefore this
PR combines the File & Directory pickers into a unified version



https://github.com/user-attachments/assets/f3bf189c-8b69-4f5f-90ce-0b83b12dbca3

(Ignore the `@mentions`, they are broken also on main)

Release Notes:

- N/A