Added local RAG (#756)

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

app/MindWork AI Studio/Tools/ContentStreamPendingContent.cs

@@ -0,0 +1,28 @@
+namespace AIStudio.Tools;
+
+/// <summary>
+/// Content which a reader held back, together with the token count of exactly that content.
+/// </summary>
+/// <remarks>
+/// Readers which assemble a page or a slide from several stream events cannot pass their content
+/// on right away. Its token count has to travel with it: the count describes the content, not the
+/// event which happened to arrive at the moment the content was released. Keeping the two together
+/// is what stops a page from being sized by the text of the page after it.
+/// </remarks>
+/// <param name="Content">The assembled content.</param>
+/// <param name="TokenCount">The number of tokens of that content, or null when it is unknown.</param>
+public readonly record struct ContentStreamPendingContent(string Content, int? TokenCount)
+{
+    /// <summary>
+    /// Adds up two token counts, where an unknown count makes the sum unknown as well.
+    /// </summary>
+    /// <remarks>
+    /// A partial sum would understate the whole and would let the chunking size a chunk by a part
+    /// of what it holds. Reporting the count as unknown is the honest answer, because the caller
+    /// can still count the content itself.
+    /// </remarks>
+    /// <param name="left">The first count, or null when it is unknown.</param>
+    /// <param name="right">The second count, or null when it is unknown.</param>
+    /// <returns>The sum, or null when either count is unknown.</returns>
+    public static int? AddTokenCounts(int? left, int? right) => left is null || right is null ? null : left + right;
+}