Let the sources of your own documents open at the page they were found on (#974)

app/Tests/Tools/FileExportFormatTests.cs

@@ -0,0 +1,39 @@
+using AIStudio.Tools;
+
+namespace AIStudio.Tests.Tools;
+
+/// <summary>
+/// Checks what AI Studio assumes about the readers of the formats it writes.
+/// </summary>
+[TestFixture]
+public sealed class FileExportFormatTests
+{
+    [Test]
+    public void OnlyTheTwoOfficeFormatsRefuseAPageInALocalLink()
+    {
+        Assert.Multiple(() =>
+        {
+            Assert.That(FileExportFormat.MICROSOFT_WORD.FollowsPageAnchors(), Is.False, "Word looks for a file whose name ends in the fragment, finds none, and refuses the link.");
+            Assert.That(FileExportFormat.OPEN_DOCUMENT_TEXT.FollowsPageAnchors(), Is.False, "LibreOffice does the same, verified on 2026-09-15 with an exported .odt.");
+            Assert.That(FileExportFormat.HTML.FollowsPageAnchors(), Is.True, "A browser opens the document on the page the fragment names.");
+            Assert.That(FileExportFormat.MARKDOWN.FollowsPageAnchors(), Is.True);
+            Assert.That(FileExportFormat.LATEX.FollowsPageAnchors(), Is.True);
+        });
+    }
+
+    [Test]
+    public void EveryFormatAnAnswerIsWrittenAsHasAnAnswerHere()
+    {
+        // Whoever adds a format decides what its reader can follow, rather than inheriting an
+        // assumption. This fails for a format which nobody thought about, because the list below
+        // has to name it:
+        Assert.That(FileExportFormatExtensions.ANSWER_FORMATS, Is.EquivalentTo(new[]
+        {
+            FileExportFormat.MICROSOFT_WORD,
+            FileExportFormat.OPEN_DOCUMENT_TEXT,
+            FileExportFormat.LATEX,
+            FileExportFormat.MARKDOWN,
+            FileExportFormat.HTML,
+        }), "A format was added to or removed from the export menu: say in FollowsPageAnchors whether its reader follows a page in a local link, then name it here.");
+    }
+}