.NET - Introduce ChatCompletionAgent Allow-List of supported content (#10350)

### Motivation and Context
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Ensure `ChatCompletionAgent` only accepts content types / patterns
allowed by chat-completion service.

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

The sample
[Concepts/Agents/MixedChat_Images](https://github.com/microsoft/semantic-kernel/blob/main/dotnet/samples/Concepts/Agents/MixedChat_Images.cs)
fails (connector assertion) when passing a message generated by
`OpenAIAssistantAgent` to `ChatCompletionAgent` that contains only
`FileReferenceContent`.

> This sample historically worked, but there may have been an internal
logic update for the connector as part of a maintenance task. I didn't
bother to do the forensics, as the fact that it fails now is sufficient
data to address the bug.

### 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 😄

Author: crickman

dotnet/src/Agents/Core/ChatHistoryChannel.cs

@@ -1,4 +1,5 @@
 // Copyright (c) Microsoft. All rights reserved.
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
@@ -16,6 +17,15 @@ namespace Microsoft.SemanticKernel.Agents;
 /// </summary>
 internal sealed class ChatHistoryChannel : AgentChannel
 {
+    // Supported content types for <see cref="ReceiveAsync"/> when
+    // <see cref="ChatMessageContent.Content"/> is empty.
+    private static readonly HashSet<Type> s_contentMap =
+        [
+            typeof(FunctionCallContent),
+            typeof(FunctionResultContent),
+            typeof(ImageContent),
+        ];
+
     private readonly ChatHistory _history;
 
     /// <inheritdoc/>
@@ -105,7 +115,11 @@ protected override async IAsyncEnumerable<StreamingChatMessageContent> InvokeStr
     /// <inheritdoc/>
     protected override Task ReceiveAsync(IEnumerable<ChatMessageContent> history, CancellationToken cancellationToken)
     {
-        this._history.AddRange(history);
+        // Only add messages with valid content or supported content-items.
+        this._history.AddRange(
+            history.Where(
+                m => !string.IsNullOrEmpty(m.Content) ||
+                m.Items.Where(i => s_contentMap.Contains(i.GetType())).Any()));
 
         return Task.CompletedTask;
     }

dotnet/src/Agents/UnitTests/Core/ChatHistoryChannelTests.cs

@@ -1,8 +1,10 @@
 // Copyright (c) Microsoft. All rights reserved.
+using System;
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.SemanticKernel;
 using Microsoft.SemanticKernel.Agents;
+using Microsoft.SemanticKernel.ChatCompletion;
 using Moq;
 using Xunit;
 
@@ -18,13 +20,103 @@ public class ChatHistoryChannelTests
     /// does not implement <see cref="ChatHistoryKernelAgent"/>.
     /// </summary>
     [Fact]
-    public async Task VerifyAgentWithoutIChatHistoryHandlerAsync()
+    public async Task VerifyAgentIsChatHistoryKernelAgentAsync()
     {
         // Arrange
         Mock<Agent> agent = new(); // Not a ChatHistoryKernelAgent
-        ChatHistoryChannel channel = new(); // Requires IChatHistoryHandler
+        ChatHistoryChannel channel = new();
 
         // Act & Assert
         await Assert.ThrowsAsync<KernelException>(() => channel.InvokeAsync(agent.Object).ToArrayAsync().AsTask());
     }
+
+    /// <summary>
+    /// Verify a <see cref="ChatHistoryChannel"/> filters empty content on receive.
+    /// </summary>
+    [Fact]
+    public async Task VerifyReceiveFiltersEmptyContentAsync()
+    {
+        // Arrange
+        ChatHistoryChannel channel = new();
+
+        // Act
+        await channel.ReceiveAsync([new ChatMessageContent(AuthorRole.Assistant, string.Empty)]);
+
+        // Assert
+        Assert.Empty(await channel.GetHistoryAsync().ToArrayAsync());
+    }
+
+    /// <summary>
+    /// Verify a <see cref="ChatHistoryChannel"/> filters file content on receive.
+    /// </summary>
+    /// <remarks>
+    /// As long as content is not empty, extraneous file content is ok.
+    /// </remarks>
+    [Fact]
+    public async Task VerifyReceiveFiltersFileContentAsync()
+    {
+        // Arrange
+        ChatHistoryChannel channel = new();
+
+        // Act
+        await channel.ReceiveAsync([new ChatMessageContent(AuthorRole.Assistant, [new FileReferenceContent("fileId")])]);
+
+        // Assert
+        Assert.Empty(await channel.GetHistoryAsync().ToArrayAsync());
+
+        // Act
+        await channel.ReceiveAsync(
+            [new ChatMessageContent(
+                AuthorRole.Assistant,
+                [
+                    new TextContent("test"),
+                    new FileReferenceContent("fileId")
+                ])]);
+
+        // Assert
+        var history = await channel.GetHistoryAsync().ToArrayAsync();
+        Assert.Single(history);
+        Assert.Equal(2, history[0].Items.Count);
+    }
+
+    /// <summary>
+    /// Verify a <see cref="ChatHistoryChannel"/> accepts function content on receive.
+    /// </summary>
+    [Fact]
+    public async Task VerifyReceiveAcceptsFunctionContentAsync()
+    {
+        // Arrange
+        ChatHistoryChannel channel = new();
+
+        // Act
+        await channel.ReceiveAsync([new ChatMessageContent(AuthorRole.Assistant, [new FunctionCallContent("test-func")])]);
+
+        // Assert
+        Assert.Single(await channel.GetHistoryAsync().ToArrayAsync());
+
+        // Arrange
+        channel = new();
+
+        // Act
+        await channel.ReceiveAsync([new ChatMessageContent(AuthorRole.Assistant, [new FunctionResultContent("test-func")])]);
+
+        // Assert
+        Assert.Single(await channel.GetHistoryAsync().ToArrayAsync());
+    }
+
+    /// <summary>
+    /// Verify a <see cref="ChatHistoryChannel"/> accepts image content on receive.
+    /// </summary>
+    [Fact]
+    public async Task VerifyReceiveAcceptsImageContentAsync()
+    {
+        // Arrange
+        ChatHistoryChannel channel = new();
+
+        // Act
+        await channel.ReceiveAsync([new ChatMessageContent(AuthorRole.Assistant, [new ImageContent(new Uri("http://test.ms/test.jpg"))])]);
+
+        // Assert
+        Assert.Single(await channel.GetHistoryAsync().ToArrayAsync());
+    }
 }