Rework of the file export (#894)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Tools/MessageTable.cs

@@ -0,0 +1,12 @@
+namespace AIStudio.Tools;
+
+/// <summary>
+/// A table found in a message, ready to be written to a file.
+/// </summary>
+/// <param name="Ordinal">Which table of the message this is, counting from one. The same table
+/// appears once per format we offer for it, so this is what tells two tables apart even when they
+/// carry the same heading.</param>
+/// <param name="Caption">What the table is about, taken from its first column heading.</param>
+/// <param name="Format">The format this content is written as.</param>
+/// <param name="Content">The finished file content.</param>
+public sealed record MessageTable(int Ordinal, string Caption, FileExportFormat Format, string Content);