.NET: Preserve opening user message in history during compaction tool-call loops

[banned2054]

Motivation & Context

When a CompactionProvider runs inside a function-calling loop, the opening user message can be omitted from InMemoryChatHistoryProvider even when the compaction threshold is never reached. On the second model call, the provider marks previously indexed messages as ChatHistory. Because those messages share instances with the original request, the history provider then filters out the opening user message when persisting the run.

This change preserves the opening request alongside the tool call, tool result, and final response.

Description & Review Guide

• What are the major changes? Clone input messages and their AdditionalProperties dictionaries before adding them to the compaction index, so source attribution changes do not mutate the caller's messages. Add a provider-level mutation regression test and full tool-loop regression tests for both RunAsync and RunStreamingAsync.
• What is the impact of these changes? The opening user message is persisted correctly when compaction is enabled and the first model response is a tool call. No public API or dependency changes. Each provider invocation allocates message and property-dictionary copies; message contents remain shared.
• What do you want reviewers to focus on? Isolation of source attribution, including messages with existing AdditionalProperties. The tool-loop tests verify two model calls, a tool result in the second request, and the exact persisted sequence: user → assistant(tool call) → tool(result) → assistant(final). Both tests fail against the original implementation with the opening user message missing and pass with the fix.

Related Issue

Fixes #8441

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.

[Copilot]

🟢 Approval recommended

The focused fix correctly isolates message metadata and includes comprehensive regression coverage for both execution paths.

Pull request overview

Fixes #8441 by isolating compaction state from caller-owned messages, preserving the opening user message during tool-call loops.

Changes:

• Clone messages and property dictionaries before compaction indexing.
• Add mutation and sync/streaming tool-loop regression tests.

File summaries

File	Description
dotnet/src/Microsoft.Agents.AI/Compaction/CompactionProvider.cs	Prevents source-attribution mutations from leaking to input messages.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/CompactionProviderTests.cs	Tests input-message isolation.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ChatHistoryManagementTests.cs	Tests complete history persistence for synchronous and streaming tool loops.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced

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[banned2054]

@microsoft-github-policy-service agree