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

app/Tests/Assistants/ChatSystemPromptChoiceTests.cs

@@ -0,0 +1,49 @@
+using AIStudio.Assistants;
+using AIStudio.Chat;
+
+namespace AIStudio.Tests.Assistants;
+
+/// <summary>
+/// Checks which system prompt a chat gets for each choice an assistant can make.
+/// </summary>
+[TestFixture]
+public sealed class ChatSystemPromptChoiceTests
+{
+    private const string PROMPT_OF_ASSISTANT = "You are a translation engine. Only translate the text.";
+    private const string PROMPT_FOR_THE_CHAT = "You are an expert in German and answer questions about a translation.";
+
+    [Test]
+    public void TheDefaultChoiceGivesTheDefaultPromptOfAIStudio() => Assert.That(ChatSystemPromptChoice.AI_STUDIO_DEFAULT.CreatePrompt(PROMPT_OF_ASSISTANT), Is.EqualTo(SystemPrompts.DEFAULT));
+
+    [Test]
+    public void TheAssistantChoiceGivesThePromptOfTheAssistant() => Assert.That(ChatSystemPromptChoice.ASSISTANT.CreatePrompt(PROMPT_OF_ASSISTANT), Is.EqualTo(PROMPT_OF_ASSISTANT));
+
+    [Test]
+    public void ACustomChoiceGivesThePromptTheAssistantWroteForTheChat()
+    {
+        var choice = ChatSystemPromptChoice.Custom(() => PROMPT_FOR_THE_CHAT);
+
+        using (Assert.EnterMultipleScope())
+        {
+            Assert.That(choice.Source, Is.EqualTo(ChatSystemPromptSource.CUSTOM));
+            Assert.That(choice.CreatePrompt(PROMPT_OF_ASSISTANT), Is.EqualTo(PROMPT_FOR_THE_CHAT));
+        }
+    }
+
+    [Test]
+    public void ACustomPromptIsBuiltOnlyWhenTheChatNeedsIt()
+    {
+        var builds = 0;
+        var choice = ChatSystemPromptChoice.Custom(() =>
+        {
+            builds++;
+            return PROMPT_FOR_THE_CHAT;
+        });
+
+        Assert.That(choice.Source, Is.EqualTo(ChatSystemPromptSource.CUSTOM));
+        Assert.That(builds, Is.Zero, "Reading the choice must not build the prompt, which may need the state of the assistant.");
+
+        choice.CreatePrompt(PROMPT_OF_ASSISTANT);
+        Assert.That(builds, Is.EqualTo(1));
+    }
+}