.Net: Add ChatReducerTriggerEvent with AfterToolCallResponseReceived support

[Copilot]

Motivation and Context

Long-running agentic workflows can invoke 50+ tool calls in a single turn, exhausting model token limits before completion. Existing chat history reducers only trigger before sending messages to the provider, making them unable to trim history during multi-step tool execution chains.

Description

Implements trigger-based chat history reduction with a new AfterToolCallResponseReceived event that fires after each FunctionResultContent is received, enabling incremental history management during tool-heavy agent workflows.

Core Infrastructure:

• ChatReducerTriggerEvent enum: AfterMessageAdded, BeforeMessagesRetrieval, AfterToolCallResponseReceived
• IChatHistoryReducerWithTrigger: Extends IChatHistoryReducer with trigger configuration
• ChatHistoryReducerBase: Abstract base class with trigger logic and defaults to BeforeMessagesRetrieval

Agent Integration:

• ChatHistoryAgent.ReduceAsync(ChatHistory, ChatReducerTriggerEvent, CancellationToken): Trigger-aware reduction
• ChatHistoryChannel: Invokes reducer with AfterToolCallResponseReceived when detecting FunctionResultContent

Usage:

// Reducer that trims history after each tool call response
public class ToolCallReducer : ChatHistoryReducerBase
{
    public ToolCallReducer() 
        : base(ChatReducerTriggerEvent.AfterToolCallResponseReceived) { }
    
    public override Task<IEnumerable<ChatMessageContent>?> ReduceAsync(
        IReadOnlyList<ChatMessageContent> history, 
        CancellationToken ct = default)
    {
        // Keep only last 20 messages
        return Task.FromResult<IEnumerable<ChatMessageContent>?>(
            history.Count > 20 ? history.Skip(history.Count - 20).ToList() : null);
    }
}

// Configure agent with trigger-aware reducer
var agent = new ChatCompletionAgent
{
    HistoryReducer = new ToolCallReducer()
};

Backward compatible: Reducers implementing only IChatHistoryReducer continue to work with existing BeforeMessagesRetrieval behavior.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.Net: New Feature: Add New ChatReducerTriggerEvent : AfterToolCallResponseReceived</issue_title>
<issue_description>---
name: Add New ChatReducerTriggerEvent : AfterToolCallResponseReceived
about: Add a new ChatReducerTriggerEvent that will fire on each receipt of AIContent type of FunctionResultContent.

---

During long running agentic workflows, a single turn may call upwards of 50 tool calls depending on the complexity of the Agentic task. These scenarios can exceed token limits of the model provider. With a managed chat system that is utilizing InMemoryChatHistoryProvider, it would be useful to add a new ChatReducerTriggerEvent so that the chat messages may be trimmed accordingly as needed in these long running agentic use cases.

InMemoryChatHistoryProvider:
Could we add a new trigger for post tool call response received?

</issue_description>

<agent_instructions>Investigate this issue and come up with a plan to add this new feature.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes .Net: New Feature: Add New ChatReducerTriggerEvent : AfterToolCallResponseReceived #13498

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