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/eval/src/example.rs

@@ -276,7 +276,8 @@ impl ExampleContext {
                 | ThreadEvent::ReceivedTextChunk
                 | ThreadEvent::StreamedToolUse { .. }
                 | ThreadEvent::CheckpointChanged
-                | ThreadEvent::UsageUpdated(_) => {
+                | ThreadEvent::UsageUpdated(_)
+                | ThreadEvent::CancelEditing => {
                     tx.try_send(Ok(())).ok();
                     if std::env::var("ZED_EVAL_DEBUG").is_ok() {
                         println!("{}Event: {:#?}", log_prefix, event);