Added the Model Comparison assistant for blind comparisons of two models (#1009)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Assistants/ModelComparison/ModelComparisonSeriesChat.cs

@@ -0,0 +1,37 @@
+using System.Text.Json.Serialization;
+
+namespace AIStudio.Assistants.ModelComparison;
+
+/// <summary>
+/// One of the two chats of a task of a comparison series, as the series keeps it beside the chat itself.
+/// </summary>
+public sealed record ModelComparisonSeriesChat
+{
+    /// <summary>
+    /// The ID of the chat, which names its file in the folder of the series.
+    /// </summary>
+    public Guid ChatId { get; set; }
+
+    /// <summary>
+    /// When the first message of the chat was sent, in UTC, or null until the user is done with the chat. Not when the
+    /// task began: the user writes the first message only for the first chat, and the time it takes would count
+    /// against the model of that chat alone.
+    /// </summary>
+    public DateTimeOffset? StartedAtUtc { get; set; }
+
+    /// <summary>
+    /// When the user was done with the chat, in UTC, or null before.
+    /// </summary>
+    public DateTimeOffset? FinishedAtUtc { get; set; }
+
+    /// <summary>
+    /// How many messages the user wrote in the chat, the first one included.
+    /// </summary>
+    public int UserMessageCount { get; set; }
+
+    /// <summary>
+    /// How long the user worked in the chat, or null while it is not done.
+    /// </summary>
+    [JsonIgnore]
+    public TimeSpan? Duration => this.StartedAtUtc is { } started && this.FinishedAtUtc is { } finished ? finished - started : null;
+}