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,
+}