.Net: Fix to ensure OpenAIPromptExecutionSettings ChatSystemPrompt is not ignored

[markwallace-microsoft]

Motivation and Context

Resolves issue #4510

Description

Add a system message using the chat system prompt if no system message is included in the chat history

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[lil-kita]

Hi everybody @markwallace-microsoft @dmytrostruk @SergeyMenshykh
chat system prompt is still ignored

<PackageVersion Include="Microsoft.SemanticKernel" Version="1.1.0" />

// Kernel _kernel is resolved using DI

OpenAIPromptExecutionSettings settings = new();
settings.ChatSystemPrompt = "You are batman. If asked who you are, say 'I am Batman!'";
string prompt = "Who are you?";

KernelFunction function = _kernel.CreateFunctionFromPrompt(prompt, settings);
FunctionResult result = await _kernel.InvokeAsync(function, cancellationToken: cancellationToken);

string text = result.GetValue<string>();
// "I am a language model AI developed by OpenAI, designed to assist and provide information on a wide range of topics. How can I assist you today?"

It works fine in this version
<PackageVersion Include="Microsoft.SemanticKernel" Version="1.0.0-rc3" />