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/eval/src/example.rs

@@ -234,9 +234,11 @@ impl ExampleContext {
                         tx.try_send(Err(anyhow!(err.clone()))).ok();
                     }
                 },
-                ThreadEvent::StreamedAssistantText(_, _)
+                ThreadEvent::NewRequest
+                | ThreadEvent::StreamedAssistantText(_, _)
                 | ThreadEvent::StreamedAssistantThinking(_, _)
-                | ThreadEvent::UsePendingTools { .. } => {}
+                | ThreadEvent::UsePendingTools { .. }
+                | ThreadEvent::CompletionCanceled => {}
                 ThreadEvent::ToolFinished {
                     tool_use_id,
                     pending_tool_use,