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/Provider/OpenAI/ChatCompletionStreamPart.cs

@@ -5,15 +5,20 @@ namespace AIStudio.Provider.OpenAI;
 /// </summary>
 /// <param name="TextDelta">The text this line carried, empty when it carried none.</param>
 /// <param name="Sources">The sources this line announced, empty when it announced none.</param>
-public readonly record struct ChatCompletionStreamPart(string TextDelta, IList<ISource> Sources)
+/// <param name="Usage">What the provider said the request cost, unknown on every line but the one which carries it.</param>
+public readonly record struct ChatCompletionStreamPart(string TextDelta, IList<ISource> Sources, TokenUsage Usage = default)
 {
     /// <summary>
     /// The part of a line which says nothing to the user, such as a fragment of a tool call.
     /// </summary>
     public static ChatCompletionStreamPart Nothing => new(string.Empty, []);
-    
+
     /// <summary>
     /// Whether this part has anything to show at all.
     /// </summary>
+    /// <remarks>
+    /// The usage is not part of that: it is nothing to show, and whether it is passed on at all is
+    /// the adapter's decision, which knows which round this is.
+    /// </remarks>
     public bool HasContent => this.TextDelta.Length > 0 || this.Sources.Count > 0;
 }