Added the exact token count where the provider reports it

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

app/MindWork AI Studio/Provider/IResponseStreamLine.cs

@@ -16,4 +16,20 @@ public interface IResponseStreamLine : IAnnotationStreamLine
     /// </summary>
     /// <returns>The content of the response line.</returns>
     public ContentStreamChunk GetContent();
+
+    /// <summary>
+    /// Checks whether the response line states what 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.
+    /// </remarks>
+    /// <returns>True when the response line carries a token usage, false otherwise.</returns>
+    public bool ContainsUsage() => false;
+
+    /// <summary>
+    /// Gets what the provider said the request cost.
+    /// </summary>
+    /// <returns>The usage, or <see cref="TokenUsage.UNKNOWN"/> when the line carries none.</returns>
+    public TokenUsage GetUsage() => TokenUsage.UNKNOWN;
 }