Tell whoever asked for a page what the search could not cover

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

@@ -0,0 +1,25 @@
+namespace AIStudio.Tools.RAG;
+
+/// <summary>
+/// What kept a search from covering the whole data source.
+/// </summary>
+public enum RetrievalGap
+{
+    /// <summary>
+    /// The data source could not be searched at all, e.g., while it is being indexed again, or
+    /// when its ERI server could not be reached.
+    /// </summary>
+    NOT_SEARCHED,
+
+    /// <summary>
+    /// Part of the search failed, e.g., the vector search while the embedding provider is not
+    /// available. The matches came from the rest of it and may be incomplete.
+    /// </summary>
+    PARTLY_SEARCHED,
+
+    /// <summary>
+    /// The query could not be used for part of the search, e.g., because it is longer than the
+    /// embedding model accepts. A shorter query would be searched in full.
+    /// </summary>
+    QUERY_NOT_SEARCHABLE,
+}

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

@@ -19,4 +19,16 @@ public sealed record RetrievalPage(IReadOnlyList<IRetrievalContext> Contexts, bo
     /// A page without any matches and nothing after it.
     /// </summary>
     public static readonly RetrievalPage EMPTY = new([], false);
+
+    /// <summary>
+    /// What kept the search from covering the whole data source. Empty when nothing did.
+    /// </summary>
+    /// <remarks>
+    /// Without this, a data source which could not be searched would look like one which found
+    /// nothing, and the model would tell the user their documents do not mention what they might
+    /// well mention. A local data source tells the user about its own problems as well, since only
+    /// the user can fix those. Not so about problems of the query: it was written by whoever asked
+    /// for this page, and so is a better one.
+    /// </remarks>
+    public IReadOnlyList<RetrievalGap> Gaps { get; init; } = [];
 }