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/IResponseStreamLine.cs

@@ -16,4 +16,18 @@ public interface IResponseStreamLine : IAnnotationStreamLine
     /// </summary>
     /// <returns>The content of the response line.</returns>
     public ContentStreamChunk GetContent();
+
+    /// <summary>
+    /// Gets what the provider said the request cost.
+    /// </summary>
+    /// <remarks>
+    /// Answered here for every wire format which says nothing about it, which is most of them: a
+    /// provider who reports no usage is the normal case, not a gap somebody has to fill in.
+    ///
+    /// Unlike content and sources, there is no separate check for whether a line carries it. This
+    /// never fails on a line without one, and whether the answer means anything is what IsKnown of
+    /// the returned usage says.
+    /// </remarks>
+    /// <returns>The usage, or TokenUsage.UNKNOWN when the line carries none.</returns>
+    public TokenUsage GetUsage() => TokenUsage.UNKNOWN;
 }