Fix #433, #358, #342.

[lsoft]

Here is fixed:

1. The requested operation requires an element of type 'String', but the target element has type 'Object' #433 For a functions without arguments DeserializeArgumentsValue fails to deserialize.
2. ChatFinishReasonExtensions.ToChatFinishReason #342 and perhaps StreamingChatCompletionUpdate.FinishReason is always null #358 Sometimes LLMs returns invalid value for a chat finish reason (even null can returns). So preferred behavior is to return Unknown finish reason, not to raise exception.