.NET: AgentThread serialization alternatives ADR

[westey (westey-m)]

Motivation and Context

There are some drawbacks to the current serialization mechanism used for AgentThread instances, so creating an ADR to consider some alternatives and their implications.

#303

Description

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.

[Copilot]

Pull request overview

This PR introduces an Architecture Decision Record (ADR) that evaluates alternative approaches to serializing and deserializing AgentThread instances. The ADR identifies drawbacks with the current custom Serialize/Deserialize methods and proposes solutions that would enable standard serialization mechanisms like JsonSerializer.

Key Changes

• Proposes three options for AgentThread serialization: separating state from behavior with lazy reattachment, separating state from behavior completely, or maintaining the current approach
• Documents the tradeoffs between supporting custom behaviors and enabling standard serialization
• Outlines required changes to ChatMessageStore and related infrastructure to support state-based serialization