Add summary doc comment to TokenUsage class

Added a summary documentation comment to the `TokenUsage` class in the
`Microsoft.KernelMemory.Models` namespace. This comment describes the
purpose of the class, which is to represent the usage of tokens in a
request and response cycle. This improves code readability and
maintainability by providing context and explanation for the class.

Author: marcominerva

service/Abstractions/Models/TokenUsage.cs

@@ -4,6 +4,9 @@
 
 namespace Microsoft.KernelMemory.Models;
 
+/// <summary>
+/// Represents the usage of tokens in a request and response cycle.
+/// </summary>
 public class TokenUsage
 {
     /// <summary>