Fixed attached files silently reaching the AI as empty documents (#904)

app/MindWork AI Studio/Tools/ContentStreamProcessedEvent.cs

@@ -0,0 +1,23 @@
+namespace AIStudio.Tools;
+
+/// <summary>
+/// The outcome of processing one content stream event: either content to append, or a reported
+/// failure.
+/// </summary>
+/// <remarks>
+/// Content and error are kept apart on purpose. A reported failure must never be appended as
+/// content, because that would hand the failure to the AI as if it were part of the document.
+/// </remarks>
+/// <param name="Content">The content to append, or null when this event carries none.</param>
+/// <param name="Error">The reported failure, or null when the event was processed successfully.</param>
+public readonly record struct ContentStreamProcessedEvent(string? Content, ContentStreamErrorDetails? Error)
+{
+    /// <summary>
+    /// An event which neither produced content nor reported a failure.
+    /// </summary>
+    public static readonly ContentStreamProcessedEvent NOTHING = new(null, null);
+
+    public static ContentStreamProcessedEvent FromContent(string? content) => new(content, null);
+
+    public static ContentStreamProcessedEvent FromError(ContentStreamErrorDetails? error) => new(null, error);
+}