New excerpt controls (#24428)

Release Notes:

- Multibuffers now use less vertical space for excerpt boundaries.
Additionally the expand up/down arrows are hidden at the start and end
of the buffers

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Zed AI <claude-3.5-sonnet@zed.dev>

crates/assistant2/src/active_thread.rs

@@ -396,7 +396,6 @@ impl ActiveThread {
                 editor::EditorMode::AutoHeight { max_lines: 8 },
                 buffer,
                 None,
-                false,
                 window,
                 cx,
             );