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,
             );

crates/assistant2/src/assistant_panel.rs

@@ -456,13 +456,8 @@ impl AssistantPanel {
 
                 workspace.add_item_to_active_pane(
                     Box::new(cx.new(|cx| {
-                        let mut editor = Editor::for_multibuffer(
-                            buffer,
-                            Some(project.clone()),
-                            true,
-                            window,
-                            cx,
-                        );
+                        let mut editor =
+                            Editor::for_multibuffer(buffer, Some(project.clone()), window, cx);
                         editor.set_breadcrumb_header(thread_summary);
                         editor
                     })),

crates/assistant2/src/inline_assistant.rs

@@ -1341,7 +1341,7 @@ impl InlineAssistant {
                     });
 
                     enum DeletedLines {}
-                    let mut editor = Editor::for_multibuffer(multi_buffer, None, true, window, cx);
+                    let mut editor = Editor::for_multibuffer(multi_buffer, None, window, cx);
                     editor.set_soft_wrap_mode(language::language_settings::SoftWrap::None, cx);
                     editor.set_show_wrap_guides(false, cx);
                     editor.set_show_gutter(false, cx);

crates/assistant2/src/inline_prompt_editor.rs

@@ -843,7 +843,6 @@ impl PromptEditor<BufferCodegen> {
                 },
                 prompt_buffer,
                 None,
-                false,
                 window,
                 cx,
             );
@@ -1001,7 +1000,6 @@ impl PromptEditor<TerminalCodegen> {
                 },
                 prompt_buffer,
                 None,
-                false,
                 window,
                 cx,
             );