Closes https://github.com/zed-industries/zed/issues/42807

This PR makes the modal to add LLM providers a bit better to interact
with:

1. Added a scrollbar
2. Made the inputs navigable with tab
3. Added some responsiveness to ensure it resizes on shorter windows


https://github.com/user-attachments/assets/758ea5f0-6bcc-4a2b-87ea-114982f37caf

Release Notes:

- agent: Improved the modal to add LLM providers by making it responsive
and keyboard navigable.