Update CHANGELOG for v0.12.0

Author: GromNaN

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## v0.12.0 (2025-05-06)
 ### Changed
 - Changed underlying `openai/client` package version to 0.12.0
+- Add an alias for the `ClientContract` interface
 
 ## v0.10.0 (2024-06-19)
 ### Changed