Added the exact token count where the provider reports it

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

app/MindWork AI Studio/Provider/ContentStreamChunk.cs

@@ -3,9 +3,15 @@ namespace AIStudio.Provider;
 /// <summary>
 /// A chunk of content from a content stream, along with its associated sources.
 /// </summary>
+/// <remarks>
+/// The usage rides along on the chunk rather than being reported next to the stream, because a
+/// provider states it as one more line of that same stream. It is unknown on every chunk but the
+/// one which carries it, and unknown on all of them at the providers which report nothing.
+/// </remarks>
 /// <param name="Content">The text content of the chunk.</param>
 /// <param name="Sources">The list of sources associated with the chunk.</param>
-public sealed record ContentStreamChunk(string Content, IList<ISource> Sources)
+/// <param name="Usage">What the provider said the request cost, where it said anything.</param>
+public sealed record ContentStreamChunk(string Content, IList<ISource> Sources, TokenUsage Usage = default)
 {
     /// <summary>
     /// Implicit conversion to string.