.Net Agents - Fix Aggregator Streaming for Nested Mode (#9669)

### Motivation and Context
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Fixes: #8677

Aggretator agent not yielding content for streamed response when
Mode=Nested.

### Description
<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

Analyzed and addressed behavior and added integration tests.

### Contribution Checklist
<!-- Before submitting this PR, please make sure: -->

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

dotnet/src/Agents/Abstractions/AggregatorChannel.cs

@@ -54,29 +54,31 @@ protected internal override IAsyncEnumerable<ChatMessageContent> GetHistoryAsync
     /// <inheritdoc/>
     protected internal override async IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamingAsync(AggregatorAgent agent, IList<ChatMessageContent> messages, [EnumeratorCancellation] CancellationToken cancellationToken = default)
     {
-        int messageCount = await this._chat.GetChatMessagesAsync(cancellationToken).CountAsync(cancellationToken).ConfigureAwait(false);
+        int initialCount = await this._chat.GetChatMessagesAsync(cancellationToken).CountAsync(cancellationToken).ConfigureAwait(false);
 
-        if (agent.Mode == AggregatorMode.Flat)
+        await foreach (StreamingChatMessageContent message in this._chat.InvokeStreamingAsync(cancellationToken).ConfigureAwait(false))
         {
-            await foreach (StreamingChatMessageContent message in this._chat.InvokeStreamingAsync(cancellationToken).ConfigureAwait(false))
+            if (agent.Mode == AggregatorMode.Flat)
             {
                 yield return message;
             }
         }
 
         ChatMessageContent[] history = await this._chat.GetChatMessagesAsync(cancellationToken).ToArrayAsync(cancellationToken).ConfigureAwait(false);
-        if (history.Length > messageCount)
+        if (history.Length > initialCount)
         {
             if (agent.Mode == AggregatorMode.Flat)
             {
-                for (int index = messageCount; index < messages.Count; ++index)
+                for (int index = history.Length - 1; index >= initialCount; --index)
                 {
                     messages.Add(history[index]);
                 }
             }
             else if (agent.Mode == AggregatorMode.Nested)
             {
-                messages.Add(history[history.Length - 1]);
+                ChatMessageContent finalMessage = history[0]; // Order descending
+                yield return new StreamingChatMessageContent(finalMessage.Role, finalMessage.Content) { AuthorName = finalMessage.AuthorName };
+                messages.Add(finalMessage);
             }
         }
     }

dotnet/src/IntegrationTests/Agents/AggregatorAgentTests.cs

@@ -0,0 +1,184 @@
+// Copyright (c) Microsoft. All rights reserved.
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Azure.Identity;
+using Microsoft.Extensions.Configuration;
+using Microsoft.SemanticKernel;
+using Microsoft.SemanticKernel.Agents;
+using Microsoft.SemanticKernel.Agents.Chat;
+using Microsoft.SemanticKernel.ChatCompletion;
+using SemanticKernel.IntegrationTests.TestSettings;
+using xRetry;
+using Xunit;
+
+namespace SemanticKernel.IntegrationTests.Agents;
+
+#pragma warning disable xUnit1004 // Contains test methods used in manual verification. Disable warning for this file only.
+
+public sealed class AggregatorAgentTests()
+{
+    private readonly IKernelBuilder _kernelBuilder = Kernel.CreateBuilder();
+    private readonly IConfigurationRoot _configuration = new ConfigurationBuilder()
+            .AddJsonFile(path: "testsettings.json", optional: true, reloadOnChange: true)
+            .AddJsonFile(path: "testsettings.development.json", optional: true, reloadOnChange: true)
+            .AddEnvironmentVariables()
+            .AddUserSecrets<OpenAIAssistantAgentTests>()
+            .Build();
+
+    /// <summary>
+    /// Integration test for <see cref="AggregatorAgent"/> non-streamed nested response.
+    /// </summary>
+    [RetryFact(typeof(HttpOperationException))]
+    public async Task AggregatorAgentFlatResponseAsync()
+    {
+        // Arrange
+        AggregatorAgent aggregatorAgent = new(() => this.CreateChatProvider())
+        {
+            Mode = AggregatorMode.Flat,
+        };
+
+        AgentGroupChat chat = new();
+        chat.AddChatMessage(new ChatMessageContent(AuthorRole.User, "1"));
+
+        // Act
+        ChatMessageContent[] responses = await chat.InvokeAsync(aggregatorAgent).ToArrayAsync();
+
+        // Assert
+        ChatMessageContent[] innerHistory = await chat.GetChatMessagesAsync(aggregatorAgent).ToArrayAsync();
+        Assert.Equal(6, innerHistory.Length);
+        Assert.Equal(5, responses.Length);
+        Assert.NotNull(responses[4].Content);
+        AssertResponseContent(responses[4]);
+    }
+
+    /// <summary>
+    /// Integration test for <see cref="AggregatorAgent"/> non-streamed nested response.
+    /// </summary>
+    [RetryFact(typeof(HttpOperationException))]
+    public async Task AggregatorAgentNestedResponseAsync()
+    {
+        // Arrange
+        AggregatorAgent aggregatorAgent = new(() => this.CreateChatProvider())
+        {
+            Mode = AggregatorMode.Nested,
+        };
+
+        AgentGroupChat chat = new();
+        chat.AddChatMessage(new ChatMessageContent(AuthorRole.User, "1"));
+
+        // Act
+        ChatMessageContent[] responses = await chat.InvokeAsync(aggregatorAgent).ToArrayAsync();
+
+        // Assert
+        ChatMessageContent[] innerHistory = await chat.GetChatMessagesAsync(aggregatorAgent).ToArrayAsync();
+        Assert.Equal(6, innerHistory.Length);
+        Assert.Single(responses);
+        Assert.NotNull(responses[0].Content);
+        AssertResponseContent(responses[0]);
+    }
+
+    /// <summary>
+    /// Integration test for <see cref="AggregatorAgent"/> non-streamed response.
+    /// </summary>
+    [RetryFact(typeof(HttpOperationException))]
+    public async Task AggregatorAgentFlatStreamAsync()
+    {
+        // Arrange
+        AggregatorAgent aggregatorAgent = new(() => this.CreateChatProvider())
+        {
+            Mode = AggregatorMode.Flat,
+        };
+
+        AgentGroupChat chat = new();
+        chat.AddChatMessage(new ChatMessageContent(AuthorRole.User, "1"));
+
+        // Act
+        StreamingChatMessageContent[] streamedResponse = await chat.InvokeStreamingAsync(aggregatorAgent).ToArrayAsync();
+
+        // Assert
+        ChatMessageContent[] fullResponses = await chat.GetChatMessagesAsync().ToArrayAsync();
+        ChatMessageContent[] innerHistory = await chat.GetChatMessagesAsync(aggregatorAgent).ToArrayAsync();
+        Assert.NotEmpty(streamedResponse);
+        Assert.Equal(6, innerHistory.Length);
+        Assert.Equal(6, fullResponses.Length);
+        Assert.NotNull(fullResponses[0].Content);
+        AssertResponseContent(fullResponses[0]);
+    }
+
+    /// <summary>
+    /// Integration test for <see cref="AggregatorAgent"/> non-streamed response.
+    /// </summary>
+    [RetryFact(typeof(HttpOperationException))]
+    public async Task AggregatorAgentNestedStreamAsync()
+    {
+        // Arrange
+        AggregatorAgent aggregatorAgent = new(() => this.CreateChatProvider())
+        {
+            Mode = AggregatorMode.Nested,
+        };
+
+        AgentGroupChat chat = new();
+        chat.AddChatMessage(new ChatMessageContent(AuthorRole.User, "1"));
+
+        // Act
+        StreamingChatMessageContent[] streamedResponse = await chat.InvokeStreamingAsync(aggregatorAgent).ToArrayAsync();
+
+        // Assert
+        ChatMessageContent[] fullResponses = await chat.GetChatMessagesAsync().ToArrayAsync();
+        ChatMessageContent[] innerHistory = await chat.GetChatMessagesAsync(aggregatorAgent).ToArrayAsync();
+        Assert.NotEmpty(streamedResponse);
+        Assert.Equal(6, innerHistory.Length);
+        Assert.Equal(2, fullResponses.Length);
+        Assert.NotNull(fullResponses[0].Content);
+        AssertResponseContent(fullResponses[0]);
+    }
+
+    private static void AssertResponseContent(ChatMessageContent response)
+    {
+        // Counting is hard
+        Assert.True(
+            response.Content!.Contains("five", StringComparison.OrdinalIgnoreCase) ||
+            response.Content!.Contains("six", StringComparison.OrdinalIgnoreCase) ||
+            response.Content!.Contains("seven", StringComparison.OrdinalIgnoreCase) ||
+            response.Content!.Contains("eight", StringComparison.OrdinalIgnoreCase),
+            $"Content: {response}");
+    }
+
+    private AgentGroupChat CreateChatProvider()
+    {
+        // Arrange
+        AzureOpenAIConfiguration configuration = this._configuration.GetSection("AzureOpenAI").Get<AzureOpenAIConfiguration>()!;
+
+        this._kernelBuilder.AddAzureOpenAIChatCompletion(
+            configuration.ChatDeploymentName!,
+            configuration.Endpoint,
+            new AzureCliCredential());
+
+        Kernel kernel = this._kernelBuilder.Build();
+
+        ChatCompletionAgent agent =
+            new()
+            {
+                Kernel = kernel,
+                Instructions = "Your job is to count.  Always add one to the previous number and respond using the english word for that number, without explanation.",
+            };
+
+        return new AgentGroupChat(agent)
+        {
+            ExecutionSettings = new()
+            {
+                TerminationStrategy = new CountTerminationStrategy(5)
+            }
+        };
+    }
+
+    private sealed class CountTerminationStrategy(int maximumResponseCount) : TerminationStrategy
+    {
+        // Terminate when the assistant has responded N times.
+        protected override Task<bool> ShouldAgentTerminateAsync(Agent agent, IReadOnlyList<ChatMessageContent> history, CancellationToken cancellationToken)
+            => Task.FromResult(history.Count(message => message.Role == AuthorRole.Assistant) >= maximumResponseCount);
+    }
+}