assistant2: Make context pill names work like editor tabs (#22741)

Context pills for files will now only display the basename of the file.
If two files have the same base name, we will also show their parent
directories, mimicking the behavior of editor tabs.


https://github.com/user-attachments/assets/ee88ee3b-80ff-4115-9ff9-8fe4845a67d8

Note: The double `/` in the file picker is a known separate issue.

Release Notes:

- N/A

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Co-authored-by: Danilo <danilo@zed.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>