Typing: when stream is completed, delta in ChatCompletionChunk from azure openai is None; should be ChoiceDelta

Confirm this is an issue with the Python library and not an underlying OpenAI API

• This is an issue with the Python library

Describe the bug

When streaming from azure open ai API the delta of the choice is None. In the python open ai client v1.42.0 delta is type ChoiceDelta i.e. not None.

To Reproduce

Run this code in line with

    completion = await self._client.chat.completions.create(
            model=self.deployment.name,
            messages=cast(list[ChatCompletionMessageParam], messages),
            stream=True,
            temperature=temperature,
        )

    async for response_chunk in completion:
        ...

The types are:
response_chunk: ChatCompletionChunk
response_chunk.choices: list[Choice]
response_chunk.choices[0].delta: ChoiceDelta

The response from azure open ai API returns delta=Nonewhen stream ends

Response example:

Choice(delta=None, finish_reason=None ...........)

Code snippets

No response

OS

linux, ubuntu 20.04

Python version

3.12.1

Library version

openai v 1.42.0

Start by reproducing the async streaming call shown in the issue and inspect ChatCompletionChunk, Choice, and ChoiceDelta handling for the final Azure OpenAI chunk. Verify that a completed stream produces a ChoiceDelta rather than delta=None, while preserving the expected completion response behavior.