fix: typo in role from 'assitant' to 'assistant'

[AuroraZiling]

在调用的时候遇到异常，排查发现是API问题：

System.ArgumentOutOfRangeException: Unknown ChatMessageRole value. (Parameter 'value')
Actual value was assitant.
  at ChatMessageRole ChatMessageRoleExtensions.ToChatMessageRole(string value)() in ChatMessageRole.Serialization.cs:line 48
  at InternalChatCompletionStreamResponseDelta InternalChatCompletionStreamResponseDelta.DeserializeInternalChatCompletionStreamResponseDelta(JsonElement element, ModelReaderWriterOptions options)() in InternalChatCompletionStreamResponseDelta.Serialization.cs:line 159
  at InternalCreateChatCompletionStreamResponseChoice InternalCreateChatCompletionStreamResponseChoice.DeserializeInternalCreateChatCompletionStreamResponseChoice(JsonElement element, ModelReaderWriterOptions options)() in InternalCreateChatCompletionStreamResponseChoice.Serialization.cs:line 69
  at StreamingChatCompletionUpdate StreamingChatCompletionUpdate.DeserializeStreamingChatCompletionUpdate(JsonElement element, ModelReaderWriterOptions options)() in StreamingChatCompletionUpdate.Serialization.cs:line 143
  at IEnumerable<U> <>c__DisplayClass13_0<U>.<DeserializeSseToSingleViaJson>b__0(?)+(JsonElement e, ModelReaderWriterOptions o) => { }() in AsyncSseUpdateCollection.cs:line 98
  at IEnumerable<U> <>c__DisplayClass12_0<U>.<DeserializeSseToMultipleViaJson>b__0(?)+(SseItem<byte[]> item) => { }() in AsyncSseUpdateCollection.cs:line 91
  at async ValueTask<bool> AsyncSseUpdateEnumerator<U>.System.Collections.Generic.IAsyncEnumerator<U>.MoveNextAsync(?)()
  at async IAsyncEnumerable<T> AsyncSseUpdateCollection<T>.GetValuesFromPageAsync(ClientResult page)+MoveNext() x 2() in AsyncSseUpdateCollection.cs:line 78
  at async IAsyncEnumerator<T> AsyncCollectionResult<T>.GetAsyncEnumerator(CancellationToken cancellationToken)+MoveNext() x 3() in AsyncCollectionResultOfT.cs:line 29
  at async IAsyncEnumerable<ChatResponseUpdate> OpenAIChatClient.FromOpenAIStreamingChatCompletionAsync(IAsyncEnumerable<StreamingChatCompletionUpdate> updates, ChatCompletionOptions options, CancellationToken cancellationToken)+MoveNext() x 2() in OpenAIChatClient.cs:line 344
  at async IAsyncEnumerable<ChatResponseUpdate> OptimizedOpenAIApiClient.GetStreamingResponseAsync(IEnumerable<ChatMessage> messages, ChatOptions options, CancellationToken cancellationToken)+MoveNext() x 2() in AI\OpenAIKernelMixin.cs:line 85
  at async IAsyncEnumerable<StreamingChatMessageContent> ChatClientChatCompletionService.GetStreamingChatMessageContentsAsync(ChatHistory chatHistory, PromptExecutionSettings executionSettings, Kernel kernel, CancellationToken cancellationToken)+MoveNext() x 2() in ChatClientChatCompletionService.cs:line 95
  at async Task ChatService.GenerateAsync(IKernelMixin kernelMixin, ChatContext chatContext, AssistantChatMessage assistantChatMessage, CancellationToken cancellationToken) x 2() in Chat\ChatService.cs:line 330
Everywhere.Common.ChatRequestException: Unknown ChatMessageRole value. (Parameter 'value')
Actual value was assitant.