Update CHANGELOG.md

Author: andreibondarev

CHANGELOG.md

@@ -11,6 +11,7 @@
 
 ## [Unreleased]
 - [BREAKING] [https://github.com/patterns-ai-core/langchainrb/pull/894] Tools can now output image_urls, and all tool output must be wrapped by a tool_response() method
+- [BUGFIX] [https://github.com/patterns-ai-core/langchainrb/pull/921] Fix for Assistant when OpenAI o1/o3 models are used
 
 ## [0.19.3] - 2025-01-13
 - [BUGFIX] [https://github.com/patterns-ai-core/langchainrb/pull/900] Empty text content should not be set when content is nil when using AnthropicMessage