Add stray UI tweaks on the task picker (#18059)

This PR adds tiny UI tweaks to the task picker. Just making sure it is
consistent with other pickers throughout Zed.

| Before | After |
|--------|--------|
| <img width="577" alt="Screenshot 2024-09-19 at 12 07 44 PM"
src="https://github.com/user-attachments/assets/c0f010a3-6e08-47ee-9997-9df2b203977c">
| <img width="577" alt="Screenshot 2024-09-19 at 12 07 09 PM"
src="https://github.com/user-attachments/assets/74baf191-4dd9-4765-b2fd-2390d4cb31c6">
|

Release Notes:

- N/A