langchain_mistralai: Include finish_reason in response metadata when parsing MistralAI chunks toAIMessageChunk (#31667)

## Description
<!-- What does this pull request accomplish? -->
- When parsing MistralAI chunk dicts to Langchain to `AIMessageChunk`
schemas via the `_convert_chunk_to_message_chunk` utility function, the
`finish_reason` was not being included in `response_metadata` as it is
for other providers.
- This PR adds a one-liner fix to include the finish reason.

- fixes: #31666

Author: Saran33

libs/partners/mistralai/langchain_mistralai/chat_models.py

@@ -271,7 +271,8 @@ def _convert_chunk_to_message_chunk(
         if _choice.get("finish_reason") is not None and isinstance(
             chunk.get("model"), str
         ):
-            response_metadata["model_name"] = chunk.get("model")
+            response_metadata["model_name"] = chunk["model"]
+            response_metadata["finish_reason"] = _choice["finish_reason"]
         return AIMessageChunk(
             content=content,
             additional_kwargs=additional_kwargs,