Let data sources be searched page by page with a query of their own

app/MindWork AI Studio/Tools/RAG/RetrievalPage.cs

@@ -0,0 +1,22 @@
+namespace AIStudio.Tools.RAG;
+
+/// <summary>
+/// One page of what a search in a data source found.
+/// </summary>
+/// <remarks>
+/// A page does not say how many matches there are in total, and it could not: a vector search has
+/// no total, since every chunk matches, only less similar ones match less. What a page does say is
+/// whether asking for the next one is worth it.
+/// </remarks>
+/// <param name="Contexts">What this page found, the most relevant first.</param>
+/// <param name="HasMore">True when the next page can be retrieved and may hold further matches. That
+/// page can still turn out empty, when everything on it was already shown on an earlier page. False
+/// when the search is exhausted, or when this page is the last one which can be retrieved at all, cf.
+/// RetrievalPaging.GetLastPage.</param>
+public sealed record RetrievalPage(IReadOnlyList<IRetrievalContext> Contexts, bool HasMore)
+{
+    /// <summary>
+    /// A page without any matches and nothing after it.
+    /// </summary>
+    public static readonly RetrievalPage EMPTY = new([], false);
+}