Read the reported token usage in one place

app/MindWork AI Studio/Provider/IResponseStreamLine.cs

@@ -18,18 +18,16 @@ public interface IResponseStreamLine : IAnnotationStreamLine
     public ContentStreamChunk GetContent();
 
     /// <summary>
-    /// Checks whether the response line states what the request cost.
+    /// 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>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 TokenUsage.UNKNOWN when the line carries none.</returns>
     public TokenUsage GetUsage() => TokenUsage.UNKNOWN;
 }