Added the exact token count for Anthropic and OpenAI (#1004)

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

@@ -0,0 +1,18 @@
+// ReSharper disable ClassNeverInstantiated.Global
+namespace AIStudio.Provider.Anthropic;
+
+/// <summary>
+/// The message a streamed Anthropic messages call opens with, as far as it is read.
+/// </summary>
+/// <remarks>
+/// It arrives on the message start event, before any content, and states what the request
+/// carried. Its content is always empty there -- the blocks follow as events of their own -- so
+/// the usage is all there is to read.
+/// </remarks>
+public sealed record AnthropicStreamMessage
+{
+    /// <summary>
+    /// What the request carried, where the stream states it.
+    /// </summary>
+    public AnthropicUsage? Usage { get; init; }
+}