agent: Preserve chat box text on 'New From Summary' (#33220)

CC: @danilo-leal Do you have thoughts on this? I found myself typing
chat messages after a long thread and then deciding I would be better
served by restarting from a summary -- and then "poof" the contents of
my chat box was lost.

Release Notes:

- agent: "New From Summary" now preserves any unsent content in the chat
box.

crates/agent/src/message_editor.rs

@@ -240,6 +240,21 @@ impl MessageEditor {
         &self.context_store
     }
 
+    pub fn get_text(&self, cx: &App) -> String {
+        self.editor.read(cx).text(cx)
+    }
+
+    pub fn set_text(
+        &mut self,
+        text: impl Into<Arc<str>>,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        self.editor.update(cx, |editor, cx| {
+            editor.set_text(text, window, cx);
+        });
+    }
+
     pub fn expand_message_editor(
         &mut self,
         _: &ExpandMessageEditor,