.NET Compaction - DelegatingChatClient approach (#2)

[crickman]

Motivation and Context

Context management (compaction) is one of the key features in the "dev-harness" effort. This change introduces structured handling of long-running AI chat conversations by compacting historical context while preserving key decisions and intent. By reducing token growth and context drift, it improves response quality, performance, and cost predictability over extended sessions. The implementation is designed to be extensible and transparent, making context lifecycle management a first‑class concern for agent development.

Description

The goal of this approach is inject compaction as part of the IChatClient architecture.

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
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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