Added the exact token count where the provider reports it (#989)

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

app/MindWork AI Studio/Chat/ReportedTokenUsage.cs

@@ -0,0 +1,55 @@
+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 number, the model it was charged for, and the conversation it was counted on 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.
+///
+/// A plain number 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>
+    /// Which model the number was 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>
+    /// How many blocks the conversation had when the request went out, this answer included.
+    /// </summary>
+    /// <remarks>
+    /// What tells an outdated report apart without anybody having to remember anything about it.
+    /// Blocks are only ever added at the end, so while this answer is the last block, a thread with
+    /// the same count is the thread the provider saw. A lower count means an earlier message was
+    /// deleted, and that message is still inside the reported number.
+    ///
+    /// Taken when the request is sent rather than when the report arrives: whatever is deleted
+    /// while the answer streams in was still part of what the provider counted.
+    /// </remarks>
+    public int BlockCount { get; init; }
+
+    /// <summary>
+    /// States the stored number as a usage again.
+    /// </summary>
+    /// <returns>The usage, or TokenUsage.UNKNOWN when the stored number states nothing usable.</returns>
+    public TokenUsage ToTokenUsage() => TokenUsage.OfReported(this.PromptTokens);
+}