[BUG] StreamingChatCompletionUpdate.FinishReason always returns null

[BazInTheAZ]

Describe the bug

When streaming a chat completion through CompleteChatStreamingAsync, the FinishReason on the StreamingChatCompletionUpdate always returns null.

Monitoring the traffic to the API, I can see the finish reason in the returned data:

data: {"id":"2f5de4d2-34b4-4c79-a7e3-554c0cd0423e","model":"gpt-4.1-mini","created":1758571978,"object":"extensions.chat.completion.chunk","choices":[{"index":0,"delta":{"content":"!"},"end_turn":false,"finish_reason":null}],"system_fingerprint":"fp_4f3d32ad4e"}

data: {"id":"2f5de4d2-34b4-4c79-a7e3-554c0cd0423e","model":"gpt-4.1-mini","created":1758571978,"object":"extensions.chat.completion.chunk","choices":[{"index":0,"delta":{},"end_turn":true,"finish_reason":"stop"}],"system_fingerprint":"fp_4f3d32ad4e"}

data: [DONE]

Here is the relevant bit for looping through the updates:

var updates = chatClient.CompleteChatStreamingAsync(messages, options, CancellationToken.None);

await foreach (var update in updates)
{
    var token = string.Empty;
    if (update.FinishReason != null)
    {
        if (debug == "1")
        {
            token += $"\n\n----\n[DEBUG] Finish Reason: {update.FinishReason}\n----\n\n";

        }
    }
    if (update.ContentUpdate.Count == 0) continue;
    token += update.ContentUpdate[0].Text;
    if (string.IsNullOrEmpty(token)) continue;
    
    
    var bytes = Encoding.UTF8.GetBytes(token);
    await context.Response.OutputStream.WriteAsync(bytes, 0, bytes.Length);
    await context.Response.FlushAsync();
}

The finish reason is never added to the output. Furthermore, stepping through debugging, the FinishReason is always null.

Steps to reproduce

See code in problem description.

Code snippets

OS

Windows 11

.NET version

.NET Framework 4.8

Library version

2.4.0

[BazInTheAZ]

My mistake, sorry for the noise. ContentUpdate.Count is zero when the FinishReason is "stop".