.Net: Consider adding strict mode so that parsing invalid chat prompts throws an exception

[markwallace-microsoft]

When using a chat completion model we attempt to parse a prompt into a chat history, if this fails we always treat the entire chat prompt as a user message. If the original prompt should have parsed into a chat history but didn't due to a syntax error then a developer would want the operation to fail. We should consider a way to enable this for use cases where the prompt is expected to be using Chat ML.