.NET: Add support for IChatReducer to InMemoryChatMessageStore

[westey-m]

Motivation and Context

#295

Sample to follow if we are happy with this approach.

Description

• Update InMemoryChatMessageStore to optionally support IChatReducer
• Make InMemoryChatMessageStore public, so that users can create it with a chat reducer and provide it to the agentthread factory on ChatClientAgent.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Pull Request Overview

This PR adds optional IChatReducer support to InMemoryChatMessageStore, allowing message reduction at configurable trigger points.

• Adds IChatReducer interface with ReduceAsync method for message reduction
• Updates InMemoryChatMessageStore to support optional chat reducer with configurable trigger events
• Makes InMemoryChatMessageStore public to enable user instantiation with custom reducers

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
IChatReducer.cs	Adds new interface defining chat message reduction capability
InMemoryChatMessageStore.cs	Adds reducer support with trigger events and makes class public
InMemoryChatMessageStoreTests.cs	Adds comprehensive test coverage for new reducer functionality

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[crickman]

Have you seen this update: dotnet/extensions#6666 ?

[stephentoub]

cc: @MackinnonBuck