Kept the reported token usage as one record on the answer

app/MindWork AI Studio/Chat/ReportedTokenUsage.cs

@@ -0,0 +1,45 @@
+using AIStudio.Provider;
+
+namespace AIStudio.Chat;
+
+/// <summary>
+/// What a provider said the request behind one answer cost, as it is stored on that answer.
+/// </summary>
+/// <remarks>
+/// The numbers and the model they were charged for travel as one value. Each of them is meaningless
+/// without the others, and loose fields on the answer could be set, cleared, or copied apart.
+///
+/// Plain numbers rather than a TokenUsage: that type only ever comes out of its own factory, which
+/// is what keeps an impossible usage from existing, while a stored value has to be readable back by
+/// the serializer. ToTokenUsage is the way back, and it treats a chat file somebody edited by hand
+/// the same way the reading side treats a provider's JSON.
+/// </remarks>
+public sealed record ReportedTokenUsage
+{
+    /// <summary>
+    /// What everything sent to the model cost.
+    /// </summary>
+    public int PromptTokens { get; init; }
+
+    /// <summary>
+    /// What the model wrote in answer.
+    /// </summary>
+    public int CompletionTokens { get; init; }
+
+    /// <summary>
+    /// Which model the numbers were charged for.
+    /// </summary>
+    /// <remarks>
+    /// A token count belongs to the tokenizer which produced it. Switch the model of a chat, and
+    /// the same conversation is worth a different number of tokens -- so the reported one stops
+    /// being an answer about the request which is about to be sent, and the estimate, wrong as it
+    /// is, is at least wrong about the right model.
+    /// </remarks>
+    public string ModelId { get; init; } = string.Empty;
+
+    /// <summary>
+    /// States the stored numbers as a usage again.
+    /// </summary>
+    /// <returns>The usage, or TokenUsage.UNKNOWN when the stored numbers state nothing usable.</returns>
+    public TokenUsage ToTokenUsage() => TokenUsage.OfReported(this.PromptTokens, this.CompletionTokens);
+}