Add CHANGELOG entry for async background callback fix (#3822)

Author: imurraywano

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 - [#3805](https://github.com/plotly/dash/pull/3805) Fix FastAPI POST routes deadlock caused by middleware consuming request body. Fixes [#3801](https://github.com/plotly/dash/issues/3801).
+- [#3822](https://github.com/plotly/dash/pull/3822) Fix `UnboundLocalError` for `user_callback_output` in async background callbacks (Celery and Diskcache managers) when the callback raises `PreventUpdate` or another exception before the variable is assigned.
 
 ## [4.2.0] - 2026-06-01 - *The Freedom Update*