agent: Review edits in single-file editors (#29820)

Enables reviewing agent edits from single-file editors in addition to
the multibuffer experience we already had.


https://github.com/user-attachments/assets/a2c287f0-51d6-43a1-8537-821498b91983


This feature can be turned off by setting `assistant.single_file_review:
false`.

Release Notes:

- agent: Review edits in single-file editors

crates/workspace/src/toolbar.rs

@@ -111,6 +111,7 @@ impl Render for Toolbar {
             })
             .border_b_1()
             .border_color(cx.theme().colors().border_variant)
+            .relative()
             .bg(cx.theme().colors().toolbar_background)
             .when(has_left_items || has_right_items, |this| {
                 this.child(