Update changelog for integrations/mistral (#2300)

Co-authored-by: sjrl <10526848+sjrl@users.noreply.github.com>

Author: HaystackBot

integrations/mistral/CHANGELOG.md

@@ -1,5 +1,23 @@
 # Changelog
 
+## [integrations/mistral-v0.3.1] - 2025-09-23
+
+### 🐛 Bug Fixes
+
+- Fix MistralChatGenerator to work with `haystack-ai>=2.18` (#2296)
+
+### 🧪 Testing
+
+- Check that `_convert_chat_completion_chunk_to_streaming_chunk` works for `MistralChatGenerator` (#1953)
+
+### 🧹 Chores
+
+- Standardize readmes - part 2 (#2205)
+
+### 🌀 Miscellaneous
+
+- Chore: Fix linting in tests in mistral integration (#2262)
+
 ## [integrations/mistral-v0.3.0] - 2025-06-27
 
 ### 🧹 Chores