Remove assistant `ContextSnapshot` (#27822)

Motivation for this is to simplify the context types and make it cleaner
to add image context.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
d0276e6666Michael Sloan committed on 4/1/2025, 3:57:09 AM· committed by GitHubparentc729842
9 files changedLine totals unavailable