agent: Review edits in single-file editors (#29820)

Enables reviewing agent edits from single-file editors in addition to
the multibuffer experience we already had.


https://github.com/user-attachments/assets/a2c287f0-51d6-43a1-8537-821498b91983


This feature can be turned off by setting `assistant.single_file_review:
false`.

Release Notes:

- agent: Review edits in single-file editors

crates/agent/src/active_thread.rs

@@ -765,7 +765,6 @@ impl ActiveThread {
                     .unwrap()
             }
         });
-
         let mut this = Self {
             language_registry,
             thread_store,
@@ -954,6 +953,9 @@ impl ActiveThread {
             ThreadEvent::UsageUpdated(usage) => {
                 self.last_usage = Some(*usage);
             }
+            ThreadEvent::NewRequest | ThreadEvent::CompletionCanceled => {
+                cx.notify();
+            }
             ThreadEvent::StreamedCompletion
             | ThreadEvent::SummaryGenerated
             | ThreadEvent::SummaryChanged => {