assistant2: Adjust edit files design (#27762)

This PR includes design tweaks to elements involved on the "edit files"
flow: the bar that appears above the message editor, buttons on the
multibuffer hunks, adding keybindings to the "Review Changes" button,
etc.

<img
src="https://github.com/user-attachments/assets/4bff883a-c5c4-443e-8bf5-d98f535c83ce"
width="750" />

Release Notes:

- N/A

crates/assistant2/src/active_thread.rs

@@ -1268,7 +1268,7 @@ impl ActiveThread {
 
         let editor_bg = cx.theme().colors().editor_background;
 
-        div().py_2().child(
+        div().pt_0p5().pb_2().child(
             v_flex()
                 .rounded_lg()
                 .border_1()