.Net: Added IChatHistoryReducer for ChatCompletion with AzureOpenAI and OpenAI

[sandrohanea]

Motivation and Context

When using Auto-Invocation for Tools with OpenAI and AzureOpenAI, there is no way to intercept and reduce the chat history after tools are being invoked.

This is needed as some tools might return long responses and we might want to execute different prompt compression / optimization based on the tool result.

Note: AutoFunctionInvocationFilters cannot be used as they are invoked only before the result of the tools is added to the chat history.

The alternative to this approach would be to intercept the http call to Open AI and alter the request directly but that will cause multiple allocation, serializations and deserializations and won't scale to other Connectors.

Description

This change is extracting IChatHistoryReducer from Agents.Core project and moving it to SK.Abstrations.AI.ChatCompletions.

IChatHistoryReducer was renamed to IAgentChatHistoryReducer in order to avoid confusion as it requires 2 more methods: Equals and GetHashCode that are used to compute the agent channel key. It now inherits IChatHistoryReducer.

Added tests to ensure that a DI registered reducer is used in the ChatCompletionService of both OpenAI and AzureOpenAI connectors.

Important notice: the reducer is not mutating the ChatHistory, as it might be that new tool invocation or chat turns will require different reducing logic. (e.g. context-aware summarization)

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 😄

[crickman]

Would it make sense to provide both implementations of the reducer in the core framework:

• Truncation
• Summarization