.Net: Enable clients to remove safe_prompt attribute from json (#12631)

### Motivation and Context

Solves: #12629

### Description

See linked issue

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [😓] I didn't break anyone 😄

Co-authored-by: Marcus Kohnert <[email protected]>

Author: MarcusKohnert

dotnet/src/Connectors/Connectors.MistralAI/Client/ChatCompletionRequest.cs

@@ -30,7 +30,8 @@ internal sealed class ChatCompletionRequest
     public bool Stream { get; set; } = false;
 
     [JsonPropertyName("safe_prompt")]
-    public bool SafePrompt { get; set; } = false;
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public bool? SafePrompt { get; set; } = false;
 
     [JsonPropertyName("tools")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]

dotnet/src/Connectors/Connectors.MistralAI/MistralAIPromptExecutionSettings.cs

@@ -78,7 +78,7 @@ public int? MaxTokens
     /// </summary>
     [JsonPropertyName("safe_prompt")]
     [JsonConverter(typeof(BoolJsonConverter))]
-    public bool SafePrompt
+    public bool? SafePrompt
     {
         get => this._safePrompt;
 
@@ -338,7 +338,7 @@ public static MistralAIPromptExecutionSettings FromExecutionSettings(PromptExecu
     private double _temperature = 0.7;
     private double _topP = 1;
     private int? _maxTokens;
-    private bool _safePrompt = false;
+    private bool? _safePrompt = false;
     private int? _randomSeed;
     private string _apiVersion = "v1";
     private MistralAIToolCallBehavior? _toolCallBehavior;