Update ChatOpenAI._astream to respect finish_reason (#9431)

Currently, ChatOpenAI._astream does not reflect finish_reason to
generation_info. Change it to reflect that.

libs/langchain/langchain/chat_models/openai.py

@@ -381,10 +381,16 @@ class ChatOpenAI(BaseChatModel):
         ):
             if len(chunk["choices"]) == 0:
                 continue
-            delta = chunk["choices"][0]["delta"]
-            chunk = _convert_delta_to_message_chunk(delta, default_chunk_class)
+            choice = chunk["choices"][0]
+            chunk = _convert_delta_to_message_chunk(
+                choice["delta"], default_chunk_class
+            )
+            finish_reason = choice.get("finish_reason")
+            generation_info = (
+                dict(finish_reason=finish_reason) if finish_reason is not None else None
+            )
             default_chunk_class = chunk.__class__
-            yield ChatGenerationChunk(message=chunk)
+            yield ChatGenerationChunk(message=chunk, generation_info=generation_info)
             if run_manager:
                 await run_manager.on_llm_new_token(chunk.content)