Ensure non-streaming usage data from function calling is in history (#5676)

It's already yielded during streaming, but it's not being surfaced for non-streaming. Do so by manufacturing a new UsageContent for the UsageDetails and adding that to the response message that's added to the history.

src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs

@@ -252,6 +252,13 @@ public partial class FunctionInvokingChatClient : DelegatingChatClient
                     }
                 }
 
+                // If the original chat completion included usage data,
+                // add that into the message so it's available in the history.
+                if (KeepFunctionCallingMessages && response.Usage is { } usage)
+                {
+                    response.Message.Contents = [.. response.Message.Contents, new UsageContent(usage)];
+                }
+
                 // Add the responses from the function calls into the history.
                 var modeAndMessages = await ProcessFunctionCallsAsync(chatMessages, options, functionCallContents, iteration, cancellationToken).ConfigureAwait(false);
                 if (modeAndMessages.MessagesAdded is not null)

test/Libraries/Microsoft.Extensions.AI.Integration.Tests/ChatClientIntegrationTests.cs

@@ -163,13 +163,25 @@ public abstract class ChatClientIntegrationTests : IDisposable
 
         int secretNumber = 42;
 
-        var response = await chatClient.CompleteAsync("What is the current secret number?", new()
+        List<ChatMessage> messages =
+        [
+            new(ChatRole.User, "What is the current secret number?")
+        ];
+
+        var response = await chatClient.CompleteAsync(messages, new()
         {
             Tools = [AIFunctionFactory.Create(() => secretNumber, "GetSecretNumber")]
         });
 
         Assert.Single(response.Choices);
         Assert.Contains(secretNumber.ToString(), response.Message.Text);
+
+        if (response.Usage is { } finalUsage)
+        {
+            UsageContent? intermediate = messages.SelectMany(m => m.Contents).OfType<UsageContent>().FirstOrDefault();
+            Assert.NotNull(intermediate);
+            Assert.True(finalUsage.TotalTokenCount > intermediate.Details.TotalTokenCount);
+        }
     }
 
     [ConditionalFact]