Added the exact token count for Anthropic

app/MindWork AI Studio/Provider/Anthropic/AnthropicStreamLine.cs

@@ -9,4 +9,14 @@ namespace AIStudio.Provider.Anthropic;
 /// <param name="Index">Which content block the event belongs to; blocks are correlated by it.</param>
 /// <param name="ContentBlock">The block as it opens, for a content block start.</param>
 /// <param name="Delta">The piece this event adds, for a content block delta or a message delta.</param>
-public readonly record struct AnthropicStreamLine(string? Type, int Index, JsonElement ContentBlock, AnthropicStreamDelta Delta);
+public readonly record struct AnthropicStreamLine(string? Type, int Index, JsonElement ContentBlock, AnthropicStreamDelta Delta)
+{
+    /// <summary>
+    /// The message the stream opens with, for a message start.
+    /// </summary>
+    /// <remarks>
+    /// Not a positional parameter, because nobody but the serializer ever builds this line with
+    /// one. It is what states the usage, for the reason given at ResponseStreamLine.GetUsage.
+    /// </remarks>
+    public AnthropicStreamMessage? Message { get; init; }
+}