.NET: Fix CompactionMessageIndex.IsSummaryMessage

[pwoosam]

Motivation & Context

Deserialized chat messages do not group summary chat messages appropriately because IsSummaryMessage will always return false when the additional property type is JsonElement.

Description & Review Guide

• What are the major changes? No major changes
• What is the impact of these changes? Supports deserialized ChatMessage.AdditionalProperties, which may be JsonElement instead of bool
• What do you want reviewers to focus on? Should IsSummaryMessage handle “bools and JsonElement” or JsonElement only?

Related Issue

Fixes #7003

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Fixes summary-message detection in the .NET compaction pipeline so assistant messages tagged with _is_summary can be recognized as CompactionGroupKind.Summary even when ChatMessage.AdditionalProperties values come back as JsonElement after deserialization.

Changes:

• Update CompactionMessageIndex.IsSummaryMessage to treat _is_summary values stored as JsonElement as summary flags.
• Add a new theory in CompactionMessageIndexTests to validate summary grouping across several _is_summary value shapes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI/Compaction/CompactionMessageIndex.cs	Adjusts summary-message detection logic to account for deserialized JsonElement values.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionMessageIndexTests.cs	Adds parameterized coverage for summary-flag handling when _is_summary is present in AdditionalProperties.

[pwoosam]

cc @westey-m

[westey-m]

Thanks for the contribution @pwoosam!