Standardize on canceled instead of cancelled (#36385)

Release Notes:

- N/A

crates/agent_ui/src/active_thread.rs

@@ -4020,7 +4020,7 @@ mod tests {
 
         cx.run_until_parked();
 
-        // Verify that the previous completion was cancelled
+        // Verify that the previous completion was canceled
         assert_eq!(cancellation_events.lock().unwrap().len(), 1);
 
         // Verify that a new request was started after cancellation