Keep data sources selectable only when their index can answer

app/MindWork AI Studio/Tools/AllowedSelectedDataSources.cs

@@ -3,11 +3,18 @@ using AIStudio.Settings;
 namespace AIStudio.Tools;
 
 /// <summary>
-/// Contains both the allowed and selected data sources.
+/// Contains the allowed and selected data sources, plus the ones waiting for their index.
 /// </summary>
 /// <remarks>
 /// The selected data sources are a subset of the allowed data sources.
+///
+/// The data sources waiting for a re-index are deliberately kept apart from the allowed ones rather
+/// than mixed in. Everything reading the allowed list -- the data source selection agent above all
+/// -- takes it to mean "may be used to answer with", and a source whose index is being rebuilt
+/// cannot answer anything. It is listed separately so the user interface can still show it and say
+/// why it is greyed out, instead of letting it vanish without a word.
 /// </remarks>
 /// <param name="AllowedDataSources">The allowed data sources.</param>
 /// <param name="SelectedDataSources">The selected data sources, which are a subset of the allowed data sources.</param>
-public readonly record struct AllowedSelectedDataSources(IReadOnlyList<IDataSource> AllowedDataSources, IReadOnlyList<IDataSource> SelectedDataSources);
+/// <param name="DataSourcesAwaitingReindex">The data sources which passed every check but cannot be searched until their index has been rebuilt.</param>
+public readonly record struct AllowedSelectedDataSources(IReadOnlyList<IDataSource> AllowedDataSources, IReadOnlyList<IDataSource> SelectedDataSources, IReadOnlyList<IDataSource> DataSourcesAwaitingReindex);