Fix Azure OpenAI streaming when async content filter is enabled

[frednijsvrt]

Fixes #797

This PR is an identical fix as the PR #932 and fixes an issue happening with Azure OpenAI streaming Response causing the raise of an AttributeError.

As stated on the #797, the error occurs in pydantic-ai's OpenAI model implementation where it assumes every delta in the streaming response contains a content field. However, with Azure OpenAI's API, some deltas (like role initialization) may not include content when an async content filter is enabled, resulting in choice.delta.content being None.
More info on async content filters: https://learn.microsoft.com/en-us/azure/ai-services/openai/concepts/content-filter?tabs=warning%2Cuser-prompt%2Cpython-new#asynchronous-filter

[frednijsvrt]

The liniting issue is happening because of how the Choice model is defined in the openai Python SDK: https://github.com/openai/openai-python/blob/3154a78ac8cb404d64707d63cdfe72d3db8a45be/src/openai/types/chat/chat_completion_chunk.py#L87

class Choice(BaseModel):
    delta: ChoiceDelta
    """A chat completion delta generated by streamed model responses."""

As stated in issue #797 delta can effectively be None.

[DouweM]

This will already have failed on the previous line won't it?

[frednijsvrt]

Yes, you are completely right. Sorry about this.

[frednijsvrt]

I updated this to continue to the next chunk when choice.delta is None.

[DouweM]

@frednijsvrt If necessary, you can hide the linting issue with a # pyright: ignore[reportUnnecessaryComparison] comment. Also please add a test for this scenario!

[frednijsvrt]

@DouweM I added a test and did a change to continue to the next chunk when the delta is None since all further processing was done on the delta.

[DouweM]

@frednijsvrt Thanks Fred!