Follow up for https://github.com/zed-industries/zed/pull/27058

Improves excerpt down button to only scroll when there exists lines more
than equal to `expand_excerpt_lines`. This prevents weird shift at end
of the file.

Before:


https://github.com/user-attachments/assets/244a3bd6-d813-4cc8-9dcb-3addba2b652f

After:


https://github.com/user-attachments/assets/a9a9ba62-a454-4b56-9c8a-d8e6931b270b


Release Notes:

- N/A