agent: Cancel pending in-edit user message upon new message submit (#29565)

Previously, if you clicked on a user message to edit it, and then, while
the user message has the editor pending, sent a new message via the
textarea, the whole thread would be grayed out because we hadn't
dismissed the to-be-edited pending user message. That's now fixed.

Release Notes:

- agent: Fixed a bug that would make the whole thread be grayed out upon
sending a new message while a user message had a pending edit.

crates/agent/src/message_editor.rs

@@ -244,6 +244,10 @@ impl MessageEditor {
             return;
         }
 
+        self.thread.update(cx, |thread, cx| {
+            thread.cancel_editing(cx);
+        });
+
         if self.thread.read(cx).is_generating() {
             self.stop_current_and_send_new_message(window, cx);
             return;
@@ -312,6 +316,10 @@ impl MessageEditor {
     }
 
     fn stop_current_and_send_new_message(&mut self, window: &mut Window, cx: &mut Context<Self>) {
+        self.thread.update(cx, |thread, cx| {
+            thread.cancel_editing(cx);
+        });
+
         let cancelled = self.thread.update(cx, |thread, cx| {
             thread.cancel_last_completion(Some(window.window_handle()), cx)
         });