.Net: Moved IChatHistoryReducer from Agents to SK packages (#10285)

### Motivation and Context

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Resolves: #10203

This PR:
- Moves `IChatHistoryReducer` from `Agents.Core` to
`Microsoft.SemanticKernel.Abstractions` package.
- Moves implementations `ChatHistorySummarizationReducer` and
`ChatHistoryTruncationReducer` from `Agents.Core` to
`Microsoft.SemanticKernel.Core` package.

The functionality is marked as experimental.

### 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: dmytrostruk

dotnet/samples/Concepts/Agents/ChatCompletion_HistoryReducer.cs

@@ -1,7 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
+
 using Microsoft.SemanticKernel;
 using Microsoft.SemanticKernel.Agents;
-using Microsoft.SemanticKernel.Agents.History;
 using Microsoft.SemanticKernel.ChatCompletion;
 
 namespace Agents;

dotnet/samples/Concepts/ChatCompletion/ChatHistoryReducers/ChatHistoryExtensions.cs

@@ -19,7 +19,7 @@ internal static class ChatHistoryExtensions
     /// <remarks>
     /// For simplicity only a single system message is supported in these examples.
     /// </remarks>
-    internal static ChatMessageContent? GetSystemMessage(this ChatHistory chatHistory)
+    internal static ChatMessageContent? GetSystemMessage(this IReadOnlyList<ChatMessageContent> chatHistory)
     {
         return chatHistory.FirstOrDefault(m => m.Role == AuthorRole.System);
     }
@@ -34,7 +34,9 @@ internal static class ChatHistoryExtensions
     /// <param name="summaryMessage">An optional summary messageContent to include</param>
     /// <param name="messageFilter">An optional message filter</param>
     public static IEnumerable<ChatMessageContent> Extract(
-        this ChatHistory chatHistory, int startIndex, int? endIndex = null,
+        this IReadOnlyList<ChatMessageContent> chatHistory,
+        int startIndex,
+        int? endIndex = null,
         ChatMessageContent? systemMessage = null,
         ChatMessageContent? summaryMessage = null,
         Func<ChatMessageContent, bool>? messageFilter = null)
@@ -71,11 +73,11 @@ public static IEnumerable<ChatMessageContent> Extract(
     /// <summary>
     /// Compute the index truncation where truncation should begin using the current truncation threshold.
     /// </summary>
-    /// <param name="chatHistory">ChatHistory instance to be truncated</param>
-    /// <param name="truncatedSize"></param>
-    /// <param name="truncationThreshold"></param>
+    /// <param name="chatHistory">The source history.</param>
+    /// <param name="truncatedSize">Truncated size.</param>
+    /// <param name="truncationThreshold">Truncation threshold.</param>
     /// <param name="hasSystemMessage">Flag indicating whether or not the chat history contains a system messageContent</param>
-    public static int ComputeTruncationIndex(this ChatHistory chatHistory, int truncatedSize, int truncationThreshold, bool hasSystemMessage)
+    public static int ComputeTruncationIndex(this IReadOnlyList<ChatMessageContent> chatHistory, int truncatedSize, int truncationThreshold, bool hasSystemMessage)
     {
         if (chatHistory.Count <= truncationThreshold)
         {

dotnet/samples/Concepts/ChatCompletion/ChatHistoryReducers/MaxTokensChatHistoryReducer.cs renamed to dotnet/samples/Concepts/ChatCompletion/ChatHistoryReducers/ChatHistoryMaxTokensReducer.cs

@@ -11,15 +11,15 @@ namespace ChatCompletion;
 /// <remarks>
 /// This reducer requires that the ChatMessageContent.MetaData contains a TokenCount property.
 /// </remarks>
-public sealed class MaxTokensChatHistoryReducer : IChatHistoryReducer
+public sealed class ChatHistoryMaxTokensReducer : IChatHistoryReducer
 {
     private readonly int _maxTokenCount;
 
     /// <summary>
-    /// Creates a new instance of <see cref="MaxTokensChatHistoryReducer"/>.
+    /// Creates a new instance of <see cref="ChatHistoryMaxTokensReducer"/>.
     /// </summary>
     /// <param name="maxTokenCount">Max token count to send to the model.</param>
-    public MaxTokensChatHistoryReducer(int maxTokenCount)
+    public ChatHistoryMaxTokensReducer(int maxTokenCount)
     {
         if (maxTokenCount <= 0)
         {
@@ -30,7 +30,7 @@ public MaxTokensChatHistoryReducer(int maxTokenCount)
     }
 
     /// <inheritdoc/>
-    public Task<IEnumerable<ChatMessageContent>?> ReduceAsync(ChatHistory chatHistory, CancellationToken cancellationToken = default)
+    public Task<IEnumerable<ChatMessageContent>?> ReduceAsync(IReadOnlyList<ChatMessageContent> chatHistory, CancellationToken cancellationToken = default)
     {
         var systemMessage = chatHistory.GetSystemMessage();
 
@@ -47,12 +47,13 @@ public MaxTokensChatHistoryReducer(int maxTokenCount)
     }
 
     #region private
+
     /// <summary>
     /// Compute the index truncation where truncation should begin using the current truncation threshold.
     /// </summary>
-    /// <param name="chatHistory">ChatHistory instance to be truncated</param>
+    /// <param name="chatHistory">Chat history to be truncated.</param>
     /// <param name="systemMessage">The system message</param>
-    private int ComputeTruncationIndex(ChatHistory chatHistory, ChatMessageContent? systemMessage)
+    private int ComputeTruncationIndex(IReadOnlyList<ChatMessageContent> chatHistory, ChatMessageContent? systemMessage)
     {
         var truncationIndex = -1;
 
@@ -83,5 +84,6 @@ private int ComputeTruncationIndex(ChatHistory chatHistory, ChatMessageContent?
 
         return truncationIndex;
     }
+
     #endregion
 }

dotnet/samples/Concepts/ChatCompletion/ChatHistoryReducers/ChatHistoryReducerTests.cs

@@ -11,31 +11,6 @@ namespace ChatCompletion;
 /// </summary>
 public class ChatHistoryReducerTests(ITestOutputHelper output) : BaseTest(output)
 {
-    [Theory]
-    [InlineData(3, null, null, 5, 0)]
-    [InlineData(2, null, null, 1, 1)]
-    [InlineData(2, "SystemMessage", null, 2, 2)]
-    [InlineData(10, null, null, 3, 3)]
-    [InlineData(10, "SystemMessage", null, 3, 3)]
-    [InlineData(9, null, null, 5, 5)]
-    [InlineData(11, null, null, 5, 5)]
-    [InlineData(8, "SystemMessage", null, 5, 5)]
-    [InlineData(10, "SystemMessage", null, 5, 5)]
-    [InlineData(3, null, new int[] { 0 }, 3, 2)]
-    [InlineData(3, "SystemMessage", new int[] { 0 }, 4, 3)]
-    public async Task VerifyTruncatingChatHistoryReducerAsync(int messageCount, string? systemMessage, int[]? functionCallIndexes, int truncatedSize, int expectedSize)
-    {
-        // Arrange
-        var chatHistory = CreateHistoryWithUserInput(messageCount, systemMessage, functionCallIndexes);
-        var reducer = new TruncatingChatHistoryReducer(truncatedSize);
-
-        // Act
-        var reducedHistory = await reducer.ReduceAsync(chatHistory);
-
-        // Assert
-        VerifyReducedHistory(reducedHistory, ComputeExpectedMessages(chatHistory, expectedSize));
-    }
-
     [Theory]
     [InlineData(3, null, null, 100, 0)]
     [InlineData(3, "SystemMessage", null, 100, 0)]
@@ -47,30 +22,7 @@ public async Task VerifyMaxTokensChatHistoryReducerAsync(int messageCount, strin
     {
         // Arrange
         var chatHistory = CreateHistoryWithUserInput(messageCount, systemMessage, functionCallIndexes, true);
-        var reducer = new MaxTokensChatHistoryReducer(maxTokens);
-
-        // Act
-        var reducedHistory = await reducer.ReduceAsync(chatHistory);
-
-        // Assert
-        VerifyReducedHistory(reducedHistory, ComputeExpectedMessages(chatHistory, expectedSize));
-    }
-
-    [Theory]
-    [InlineData(3, null, null, 5, 10, 0)]
-    [InlineData(10, null, null, 5, 10, 6)]
-    [InlineData(10, "SystemMessage", null, 5, 10, 6)]
-    [InlineData(10, null, new int[] { 1 }, 5, 10, 6)]
-    [InlineData(10, "SystemMessage", new int[] { 2 }, 5, 10, 6)]
-    public async Task VerifySummarizingChatHistoryReducerAsync(int messageCount, string? systemMessage, int[]? functionCallIndexes, int truncatedSize, int truncationThreshold, int expectedSize)
-    {
-        // Arrange
-        Assert.NotNull(TestConfiguration.OpenAI.ChatModelId);
-        Assert.NotNull(TestConfiguration.OpenAI.ApiKey);
-        IChatCompletionService chatClient = new FakeChatCompletionService("The dialog consists of repetitive interaction where both the user and assistant exchange identical phrases in Latin.");
-
-        var chatHistory = CreateHistoryWithUserInput(messageCount, systemMessage, functionCallIndexes, true);
-        var reducer = new SummarizingChatHistoryReducer(chatClient, truncatedSize, truncationThreshold);
+        var reducer = new ChatHistoryMaxTokensReducer(maxTokens);
 
         // Act
         var reducedHistory = await reducer.ReduceAsync(chatHistory);