.Net: Remove default chat system prompt (#5551)

### Motivation and Context

Closes #5544 

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] 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
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

dotnet/src/Connectors/Connectors.OpenAI/AzureSdk/ClientCore.cs

@@ -772,7 +772,7 @@ internal static ChatHistory CreateNewChat(string? text = null, OpenAIPromptExecu
 
         if (!string.IsNullOrWhiteSpace(executionSettings?.ChatSystemPrompt))
         {
-            chat.AddSystemMessage(executionSettings!.ChatSystemPrompt);
+            chat.AddSystemMessage(executionSettings!.ChatSystemPrompt!);
             textRole = AuthorRole.User;
         }
 

dotnet/src/Connectors/Connectors.OpenAI/OpenAIPromptExecutionSettings.cs

@@ -174,19 +174,13 @@ public object? ResponseFormat
     /// Defaults to "Assistant is a large language model."
     /// </summary>
     [JsonPropertyName("chat_system_prompt")]
-    public string ChatSystemPrompt
+    public string? ChatSystemPrompt
     {
         get => this._chatSystemPrompt;
 
         set
         {
             this.ThrowIfFrozen();
-
-            if (string.IsNullOrWhiteSpace(value))
-            {
-                value = DefaultChatSystemPrompt;
-            }
-
             this._chatSystemPrompt = value;
         }
     }
@@ -305,11 +299,6 @@ public override PromptExecutionSettings Clone()
         };
     }
 
-    /// <summary>
-    /// Default value for chat system property.
-    /// </summary>
-    internal static string DefaultChatSystemPrompt { get; } = "Assistant is a large language model.";
-
     /// <summary>
     /// Default max tokens for a text generation
     /// </summary>
@@ -381,7 +370,7 @@ public static OpenAIPromptExecutionSettings FromExecutionSettingsWithData(Prompt
     private IDictionary<int, int>? _tokenSelectionBiases;
     private ToolCallBehavior? _toolCallBehavior;
     private string? _user;
-    private string _chatSystemPrompt = DefaultChatSystemPrompt;
+    private string? _chatSystemPrompt;
 
     #endregion
 }

dotnet/src/Connectors/Connectors.UnitTests/OpenAI/ChatCompletion/OpenAIChatCompletionServiceTests.cs

@@ -159,8 +159,8 @@ public async Task ItAddsIdToChatMessageAsync()
         var actualRequestContent = Encoding.UTF8.GetString(this._messageHandlerStub.RequestContent!);
         Assert.NotNull(actualRequestContent);
         var optionsJson = JsonSerializer.Deserialize<JsonElement>(actualRequestContent);
-        Assert.Equal(2, optionsJson.GetProperty("messages").GetArrayLength());
-        Assert.Equal("John Doe", optionsJson.GetProperty("messages")[1].GetProperty("tool_call_id").GetString());
+        Assert.Equal(1, optionsJson.GetProperty("messages").GetArrayLength());
+        Assert.Equal("John Doe", optionsJson.GetProperty("messages")[0].GetProperty("tool_call_id").GetString());
     }
 
     [Fact]
@@ -258,13 +258,10 @@ public async Task ItAddsSystemMessageAsync()
         var optionsJson = JsonSerializer.Deserialize<JsonElement>(actualRequestContent);
 
         var messages = optionsJson.GetProperty("messages");
-        Assert.Equal(2, messages.GetArrayLength());
+        Assert.Equal(1, messages.GetArrayLength());
 
-        Assert.Equal("Assistant is a large language model.", messages[0].GetProperty("content").GetString());
-        Assert.Equal("system", messages[0].GetProperty("role").GetString());
-
-        Assert.Equal("Hello", messages[1].GetProperty("content").GetString());
-        Assert.Equal("user", messages[1].GetProperty("role").GetString());
+        Assert.Equal("Hello", messages[0].GetProperty("content").GetString());
+        Assert.Equal("user", messages[0].GetProperty("role").GetString());
     }
 
     [Fact]

dotnet/src/Connectors/Connectors.UnitTests/OpenAI/OpenAIPromptExecutionSettingsTests.cs

@@ -160,7 +160,7 @@ public void ItCreatesOpenAIExecutionSettingsFromJsonSnakeCase()
     }
 
     [Theory]
-    [InlineData("", "Assistant is a large language model.")]
+    [InlineData("", "")]
     [InlineData("System prompt", "System prompt")]
     public void ItUsesCorrectChatSystemPrompt(string chatSystemPrompt, string expectedChatSystemPrompt)
     {

dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromPromptTests.cs

@@ -56,9 +56,9 @@ public void ItProvidesAccessToFunctionsViaFunctionCollection()
     }
 
     [Theory]
-    [InlineData(null, "Assistant is a large language model.")]
+    [InlineData(null, null)]
     [InlineData("My Chat Prompt", "My Chat Prompt")]
-    public async Task ItUsesChatSystemPromptWhenProvidedAsync(string? providedSystemChatPrompt, string expectedSystemChatPrompt)
+    public async Task ItUsesChatSystemPromptWhenProvidedAsync(string? providedSystemChatPrompt, string? expectedSystemChatPrompt)
     {
         // Arrange
         var mockTextGeneration = new Mock<ITextGenerationService>();

dotnet/src/SemanticKernel.UnitTests/PromptTemplate/PromptTemplateConfigTests.cs

@@ -27,8 +27,7 @@ public void DeserializingDoNotExpectChatSystemPromptToExist()
 
         // Assert
         Assert.NotNull(settings);
-        Assert.NotNull(settings.ChatSystemPrompt);
-        Assert.Equal("Assistant is a large language model.", settings.ChatSystemPrompt);
+        Assert.Null(settings.ChatSystemPrompt);
     }
 
     [Fact]