assistant2: Refine editing of previous messages (#11233)

This PR refines the UX of editing a previous message, including the
following:

- Focus the prev message body editor on double-click
- Restore previous body text on cancel
- Cancel pending completion upon submission of previous message
- Drive-by: Remove min height on composer editor

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <marshall@zed.dev>

crates/assistant2/src/ui/composer.rs

@@ -69,8 +69,7 @@ impl RenderOnce for Composer {
                                 v_flex()
                                     .justify_between()
                                     .w_full()
-                                    .gap_1()
-                                    .min_h(line_height * 4 + px(74.0))
+                                    .gap_2()
                                     .child({
                                         let settings = ThemeSettings::get_global(cx);
                                         let text_style = TextStyle {