`finish_reason` not set in AzureOpenAIChatCompletionClient.create_stream

### What happened?

The provided code snippet works fine for the `.create` call of `AzureOpenAIChatCompletionClient`, but errors on `.create_stream` call:

```
Creating client with config: {'model': 'gpt-4o', 'azure_endpoint': 'https://xxxxxxxx.openai.azure.com', 'azure_deployment': 'gpt-4o', 'api_version': '2024-08-01-preview', 'model_capabilities': {'vision': False, 'function_calling': False, 'json_output': False}, 'azure_ad_token_provider': <function get_bearer_token_provider.<locals>.wrapper at 0x108205da0>}
-----> Print output of .create call
/Users/mohammadmazraeh/Projects/autogen/python/packages/autogen-core/samples/distributed-group-chat/test_aoi.py:26: UserWarning: Resolved model mismatch: gpt-4o-2024-08-06 != gpt-4o-2024-05-13. Model mapping may be incorrect.
  single_output = await client.create(messages=messages)
-----> CreateResult(finish_reason='stop', content='The autumn leaves painted the park in vibrant shades of red and gold.', usage=RequestUsage(prompt_tokens=17, completion_tokens=14), cached=False, logprobs=None) - * 50
-----> Print output of .create_stream call
Traceback (most recent call last):
  File "/Users/mohammadmazraeh/Projects/autogen/python/packages/autogen-core/samples/distributed-group-chat/test_aoi.py", line 34, in <module>
    asyncio.run(main())
  File "/usr/local/Cellar/python@3.11/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 190, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/mohammadmazraeh/Projects/autogen/python/packages/autogen-core/samples/distributed-group-chat/test_aoi.py", line 31, in main
    async for chunk in stream_output:
  File "/Users/mohammadmazraeh/Projects/autogen/python/packages/autogen-ext/src/autogen_ext/models/_openai/_openai_client.py", line 662, in create_stream
    choice.finish_reason
AttributeError: 'NoneType' object has no attribute 'finish_reason'
```

### What did you expect to happen?

I expected to get some reasonable response from `.create_stream` call as well. 

### How can we reproduce it (as minimally and precisely as possible)?

```python
import asyncio

from autogen_core.components.models._types import UserMessage
from autogen_ext.models._openai._openai_client import AzureOpenAIChatCompletionClient
from azure.identity import DefaultAzureCredential, get_bearer_token_provider

if __name__ == "__main__":

    async def main():
        config = {
            "model": "gpt-4o",
            "azure_endpoint": "https://xxxxxxx.openai.azure.com",
            "azure_deployment": "gpt-4o",
            "api_version": "2024-08-01-preview",
            "model_capabilities": {"vision": False, "function_calling": False, "json_output": False},
            "azure_ad_token_provider": get_bearer_token_provider(
                DefaultAzureCredential(), "https://cognitiveservices.azure.com/.default"
            ),
        }

        print(f"Creating client with config: {config}")
        client = AzureOpenAIChatCompletionClient(**config)

        messages = [UserMessage(content="Generate one short sentence on some topic!", source="system")]
        print("-----> Print output of .create call")
        single_output = await client.create(messages=messages)
        print("----->", single_output, "- * 50")

        print("-----> Print output of .create_stream call")
        stream_output = client.create_stream(messages=messages)
        async for chunk in stream_output:
            print(chunk)

    asyncio.run(main())

```

### AutoGen version

0.4.0.dev6

### Which package was this bug in

Extensions

### Model used

gpt-4o

### Python version

3.11.10

### Operating system

macOS Sequoia Version 15.1 (24B83)

### Any additional info you think would be helpful for fixing this bug

_No response_

Labels: proj-extensions
State: closed