Fixed and improved the system prompt of chats started from assistants (#1172)

app/MindWork AI Studio/Assistants/ChatSystemPromptChoice.cs

@@ -0,0 +1,50 @@
+using AIStudio.Chat;
+
+namespace AIStudio.Assistants;
+
+/// <summary>
+/// Which system prompt a chat gets which continues the conversation of an assistant.
+/// </summary>
+/// <remarks>
+/// Every assistant makes this choice itself, see AssistantBase.ChatSystemPrompt. The prompt of an assistant
+/// may forbid follow-up questions or demand JSON, while the default prompt of AI Studio knows nothing about
+/// the role and the settings of the assistant. A prompt of its own for the chat keeps the role and the
+/// settings, but leaves out the rules for the result.<br/><br/>
+///
+/// That prompt is built only when it is needed, so the choice itself can be read without the state of the
+/// assistant.
+/// </remarks>
+/// <param name="Source">Where the system prompt of the chat comes from.</param>
+/// <param name="BuildCustomPrompt">Builds the prompt of the assistant for the chat; only read for ChatSystemPromptSource.CUSTOM.</param>
+public sealed record ChatSystemPromptChoice(ChatSystemPromptSource Source, Func<string> BuildCustomPrompt)
+{
+    /// <summary>
+    /// The chat gets the default prompt of AI Studio.
+    /// </summary>
+    public static readonly ChatSystemPromptChoice AI_STUDIO_DEFAULT = new(ChatSystemPromptSource.AI_STUDIO_DEFAULT, () => string.Empty);
+
+    /// <summary>
+    /// The chat gets the prompt with which the assistant ran its conversation.
+    /// </summary>
+    public static readonly ChatSystemPromptChoice ASSISTANT = new(ChatSystemPromptSource.ASSISTANT, () => string.Empty);
+
+    /// <summary>
+    /// The chat gets a prompt which the assistant writes for it.
+    /// </summary>
+    /// <param name="buildPrompt">Builds the prompt from the settings of the assistant.</param>
+    /// <returns>The choice of a prompt of its own.</returns>
+    public static ChatSystemPromptChoice Custom(Func<string> buildPrompt) => new(ChatSystemPromptSource.CUSTOM, buildPrompt);
+
+    /// <summary>
+    /// Creates the system prompt of the chat.
+    /// </summary>
+    /// <param name="promptOfAssistant">The prompt with which the assistant ran its conversation.</param>
+    /// <returns>The system prompt of the chat; empty when the assistant wrote an empty prompt for the chat.</returns>
+    public string CreatePrompt(string promptOfAssistant) => this.Source switch
+    {
+        ChatSystemPromptSource.ASSISTANT => promptOfAssistant,
+        ChatSystemPromptSource.CUSTOM => this.BuildCustomPrompt(),
+
+        _ => SystemPrompts.DEFAULT,
+    };
+}