Carry the extraction failure code into the indexer

app/MindWork AI Studio/Tools/FileExtractionException.cs

@@ -0,0 +1,27 @@
+namespace AIStudio.Tools;
+
+/// <summary>
+/// Thrown when a file could not be read, carrying the stable failure code along with the message.
+/// </summary>
+/// <remarks>
+/// The plain message alone does not say whether another attempt is worth anything. The code does,
+/// which is what the indexer needs to tell a file without readable text apart from a network drive
+/// which happens to be offline.
+/// </remarks>
+public sealed class FileExtractionException(FileExtractionErrorCode code, string message, int? pageNumber = null, string? detectedFormat = null) : Exception(message)
+{
+    /// <summary>
+    /// Gets the stable failure code.
+    /// </summary>
+    public FileExtractionErrorCode Code { get; } = code;
+
+    /// <summary>
+    /// Gets the page the failure belongs to, when the failure affects a single page only.
+    /// </summary>
+    public int? PageNumber { get; } = pageNumber;
+
+    /// <summary>
+    /// Gets the format the runtime identified by looking at the content.
+    /// </summary>
+    public string? DetectedFormat { get; } = detectedFormat;
+}