acp: Stay in edit mode when current completion ends (#36413)

When a turn ends and the checkpoint is updated, `AcpThread` emits
`EntryUpdated` with the index of the user message. This was causing the
message editor to be recreated and, therefore, lose focus.

Release Notes:

- N/A

crates/acp_thread/src/acp_thread.rs

@@ -670,6 +670,7 @@ pub struct AcpThread {
     session_id: acp::SessionId,
 }
 
+#[derive(Debug)]
 pub enum AcpThreadEvent {
     NewEntry,
     EntryUpdated(usize),