Add ADR to decide consistency of Chat History Persistence

[westey-m]

Motivation and Context

When using an AIAgent today, users get different behaviors based on whether chat history is stored service side or in a ChatHistoryProvider. This can create challenges when e.g. the function call loop is terminated and you want to resume, but you don't now whether the last FunctionResultContent is in the chat history or not.

Description

• Adding an ADR walking through the options to improve consistency

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

Adds a proposed ADR documenting the current inconsistency between service-side chat history storage and framework ChatHistoryProvider storage when tool-calling loops are involved, and frames decision drivers/options for aligning behavior.

Changes:

• Introduces ADR 0020 describing the discrepancy (persistence timing + trailing FunctionResultContent) between service-managed and provider-managed history.
• Documents two default behavior options (per-run w/ trimming vs per-service-call persistence) with pros/cons and links to related PRs/issues.