Prevent context menu "jumping" when swapping between loading state and items due to scrollbars
### Issue
When the Duo Chat context item selection menu switches between the loading state, and a list of context items which cause scrollbars to show, the whole menu "jumps". This is a bit jarring when typing as the loading state / items switch back and forward rapidly.
This issue can be seen here:

### To reproduce
Follow these testing steps to enable the context feature https://gitlab.com/gitlab-org/gitlab/-/issues/488352#note_2118028680
Open a repository with enough files to cause scrolling when searching
Open Duo Chat, type `/include` -> `Files` and type to search to trigger the issue
### Proposed solution
Improve layout so we don't get a "jump` when the scrollbar pops in and out
Investigate if the current debounce time should be increased
issue