Added prompt overrides for testing the BeyondFacts Lab (#1181)

app/MindWork AI Studio/Assistants/BeyondFactsLab/BflPromptOverrideException.cs

@@ -0,0 +1,24 @@
+namespace AIStudio.Assistants.BeyondFactsLab;
+
+/// <summary>
+/// Tells that a file in the folder for prompt overrides contains a placeholder which the lab does not fill in it.
+/// </summary>
+/// <remarks>
+/// The lab fills placeholders in the core and in the prompt of the onboarding only, and only those it knows; in any
+/// other file, a placeholder would reach the model as text. Whoever tests a changed prompt needs to learn which file
+/// and which placeholder it is, so the message names both.
+/// </remarks>
+/// <param name="fileName">The file, relative to the folder for prompt overrides, with forward slashes.</param>
+/// <param name="placeholder">The name of the placeholder, without braces.</param>
+public sealed class BflPromptOverrideException(string fileName, string placeholder) : FormatException($"The file '{fileName}' in the folder for prompt overrides contains the placeholder '{placeholder}', which the lab does not fill in this file.")
+{
+    /// <summary>
+    /// Gets the file, relative to the folder for prompt overrides, with forward slashes.
+    /// </summary>
+    public string FileName { get; } = fileName;
+
+    /// <summary>
+    /// Gets the name of the placeholder, without braces.
+    /// </summary>
+    public string Placeholder { get; } = placeholder;
+}