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/active_thread.rs

@@ -903,6 +903,11 @@ impl ActiveThread {
         cx: &mut Context<Self>,
     ) {
         match event {
+            ThreadEvent::CancelEditing => {
+                if self.editing_message.is_some() {
+                    self.cancel_editing_message(&menu::Cancel, window, cx);
+                }
+            }
             ThreadEvent::ShowError(error) => {
                 self.last_error = Some(error.clone());
             }