.NET: Add harness agent package

[westey-m]

Motivation and Context

Add package for HarnessAgent to contain a set of default capabilities

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.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 92% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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Automated review by westey-m's agents

[Copilot]

Pull request overview

This PR introduces a new .NET package, Microsoft.Agents.AI.Harness, that provides a HarnessAgent wrapper around ChatClientAgent with a preconfigured pipeline (function invocation, per-service-call chat history persistence, and context-window compaction). It also updates the Harness samples to use HarnessAgent and adds unit tests for the new agent and options.

Changes:

• Added Microsoft.Agents.AI.Harness project containing HarnessAgent, HarnessAgentOptions, and IChatClient.AsHarnessAgent(...).
• Added a new unit test project for Microsoft.Agents.AI.Harness.
• Updated Harness samples (Step01/02/03) to use AsHarnessAgent(...) instead of manually composing the pipeline.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Harness/Microsoft.Agents.AI.Harness.csproj	New packable Harness library project definition and test visibility.
dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgent.cs	Implements the preconfigured HarnessAgent pipeline and default instructions.
dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgentOptions.cs	Adds configuration options for HarnessAgent (identity, chat options, history provider, context providers).
dotnet/src/Microsoft.Agents.AI.Harness/ChatClientHarnessExtensions.cs	Adds AsHarnessAgent(...) extension on IChatClient.
dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/Microsoft.Agents.AI.Harness.UnitTests.csproj	New unit test project referencing the Harness project.
dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/HarnessAgentTests.cs	Adds unit tests for constructor validation and pipeline behavior.
dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/HarnessAgentOptionsTests.cs	Adds unit tests for HarnessAgentOptions defaults and property set/get behavior.
dotnet/samples/02-agents/Harness/Harness_Step01_Research/README.md	Updates sample description to reflect HarnessAgent usage.
dotnet/samples/02-agents/Harness/Harness_Step01_Research/Program.cs	Switches Step01 sample from manual pipeline composition to AsHarnessAgent(...).
dotnet/samples/02-agents/Harness/Harness_Step01_Research/Harness_Step01_Research.csproj	Adds project reference to the new Harness project.
dotnet/samples/02-agents/Harness/Harness_Step02_Research_WithSubAgents/README.md	Updates sample description to reflect HarnessAgent usage.
dotnet/samples/02-agents/Harness/Harness_Step02_Research_WithSubAgents/Program.cs	Switches Step02 sample agents to AsHarnessAgent(...).
dotnet/samples/02-agents/Harness/Harness_Step02_Research_WithSubAgents/Harness_Step02_Research_WithSubAgents.csproj	Adds project reference to the new Harness project.
dotnet/samples/02-agents/Harness/Harness_Step03_DataProcessing/README.md	Updates sample description to reflect HarnessAgent usage.
dotnet/samples/02-agents/Harness/Harness_Step03_DataProcessing/Program.cs	Switches Step03 sample from manual pipeline composition to AsHarnessAgent(...).
dotnet/samples/02-agents/Harness/Harness_Step03_DataProcessing/Harness_Step03_DataProcessing.csproj	Adds project reference to the new Harness project.
dotnet/agent-framework-dotnet.slnx	Adds the new Harness project and its unit tests to the solution.