Added the exact token count where the provider reports it

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/MindWork AI Studio/Provider/TokenUsage.cs

@@ -0,0 +1,81 @@
+namespace AIStudio.Provider;
+
+/// <summary>
+/// What a provider said one request actually cost, in tokens.
+/// </summary>
+/// <remarks>
+/// The counterpart to what the app counts for itself: the app estimates what the next request will
+/// cost, while this is what the provider charged for the last one. Two different statements, and
+/// this one is the only exact one of the two.
+///
+/// Nothing here says "unknown" with a zero. The default value of this type is unknown, which is the
+/// right answer for every provider which reports nothing, and a counted request can never cost zero
+/// prompt tokens because the factory below refuses to build one.
+/// </remarks>
+public readonly record struct TokenUsage
+{
+    /// <summary>
+    /// The usage of a request nobody reported anything about.
+    /// </summary>
+    public static readonly TokenUsage UNKNOWN = new();
+
+    /// <summary>
+    /// Whether a provider reported anything at all. When false, the numbers are meaningless.
+    /// </summary>
+    public bool IsKnown { get; private init; }
+
+    /// <summary>
+    /// What everything sent to the model cost: the conversation so far, its attachments, the system
+    /// prompt, and whatever tools were offered.
+    /// </summary>
+    public int PromptTokens { get; private init; }
+
+    /// <summary>
+    /// What the model wrote in answer.
+    /// </summary>
+    public int CompletionTokens { get; private init; }
+
+    /// <summary>
+    /// What the whole exchange cost, which is what the next request carries as its history.
+    /// </summary>
+    public int TotalTokens => this.PromptTokens + this.CompletionTokens;
+
+    /// <summary>
+    /// States what a provider reported.
+    /// </summary>
+    /// <remarks>
+    /// A completion of zero tokens is a real answer: a model which was cut off before writing
+    /// anything still cost its prompt. A prompt of zero is not, because there is no request
+    /// without one, and a provider sending it means we read the wrong field.
+    /// </remarks>
+    /// <param name="promptTokens">What the request carried. Has to be greater than zero.</param>
+    /// <param name="completionTokens">What the answer cost. Zero or more.</param>
+    /// <returns>The usage.</returns>
+    public static TokenUsage Of(int promptTokens, int completionTokens)
+    {
+        ArgumentOutOfRangeException.ThrowIfNegativeOrZero(promptTokens);
+        ArgumentOutOfRangeException.ThrowIfNegative(completionTokens);
+
+        return new()
+        {
+            IsKnown = true,
+            PromptTokens = promptTokens,
+            CompletionTokens = completionTokens,
+        };
+    }
+
+    /// <summary>
+    /// States what a provider reported, or unknown when it reported nothing usable.
+    /// </summary>
+    /// <remarks>
+    /// For the reading side, where the numbers come out of somebody else's JSON: a missing field, a
+    /// null, or a zero prompt all mean the same thing there, and none of them is worth an exception.
+    /// </remarks>
+    /// <param name="promptTokens">What the request carried, as the provider stated it.</param>
+    /// <param name="completionTokens">What the answer cost, as the provider stated it.</param>
+    /// <returns>The usage, or <see cref="UNKNOWN"/>.</returns>
+    public static TokenUsage OfReported(int? promptTokens, int? completionTokens) =>
+        promptTokens is > 0
+            ? Of(promptTokens.Value, completionTokens is > 0 ? completionTokens.Value : 0)
+            : UNKNOWN;
+}