add changelog

Author: SkafteNicki

src/lightning/pytorch/CHANGELOG.md

@@ -34,6 +34,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed edgecase when `max_trials` is reached in `Tuner.scale_batch_size` ([#21187](https://github.com/Lightning-AI/pytorch-lightning/pull/21187))
 
 
+- Fixed case where `LightningCLI` could not be initialized with `trainer_default` containing callbacks ([#21192](https://github.com/Lightning-AI/pytorch-lightning/pull/21192))
+
 ---
 
 ## [2.5.5] - 2025-09-05