Cover the source block of an export with tests

app/Tests/Tools/MarkdownTests.cs

@@ -0,0 +1,66 @@
+using AIStudio.Tools;
+
+namespace AIStudio.Tests.Tools;
+
+/// <summary>
+/// Checks that an answer which opens a code fence without closing it ends where it ends.
+/// </summary>
+/// <remarks>
+/// A fence without its counterpart runs to the end of the document, so whatever is appended below an
+/// answer is read as code instead of as Markdown. The chat never shows this, because it renders the
+/// answer and the sources below it as two texts. A document is one text, and there an answer which
+/// ends in an open fence takes the source list with it into a grey box.
+/// </remarks>
+[TestFixture]
+public sealed class MarkdownTests
+{
+    [Test]
+    public void AnOpenFenceGetsItsCounterpart()
+    {
+        var answer = Lines("Here is the code:", string.Empty, "```csharp", "var answer = 42;");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(Lines(answer, "```")), "The fence is closed with the same three backticks which opened it.");
+    }
+
+    [Test]
+    public void ALongerFenceIsClosedAtItsOwnLength()
+    {
+        // Four backticks are what a model writes when the block itself holds Markdown with code in
+        // it. The three backticks inside are content then, not the end of the block:
+        var answer = Lines("````markdown", "```csharp", "var answer = 42;", "```");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(Lines(answer, "````")), "Only a fence of at least the opening length closes the block.");
+    }
+
+    [Test]
+    public void ATildeFenceIsClosedWithTildes()
+    {
+        var answer = Lines("~~~", "var answer = 42;");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(Lines(answer, "~~~")), "A block opened with tildes cannot be closed with backticks.");
+    }
+
+    [Test]
+    public void AClosedFenceIsLeftAlone()
+    {
+        var answer = Lines("Here is the code:", string.Empty, "```csharp", "var answer = 42;", "```");
+
+        Assert.That(Markdown.CloseOpenCodeFence(answer), Is.EqualTo(answer), "The usual case: the model closed its block itself.");
+    }
+
+    [Test]
+    public void TextWithoutAnyFenceIsLeftAlone()
+    {
+        const string ANSWER = "Nothing in this answer opens a code block.";
+
+        Assert.That(Markdown.CloseOpenCodeFence(ANSWER), Is.EqualTo(ANSWER));
+    }
+
+    [Test]
+    public void AnEmptyTextIsLeftAlone()
+    {
+        Assert.That(Markdown.CloseOpenCodeFence(string.Empty), Is.Empty);
+    }
+
+    private static string Lines(params string[] lines) => string.Join(Environment.NewLine, lines);
+}