Update changelog

Author: cpsievert

pkg-py/CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [UNRELEASED]
+
+### Bug fixes
+
+* Only import `pydantic` if `chatlas` is relevant.
+
 ## [0.2.6] - 2025-09-11
 
 ### Bug fixes