.NET: Add background agents support to HarnessAgent

[westey-m]

Motivation and Context

Description

• Add background agents support to HarnessAgent

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 first-class “background agents” wiring to HarnessAgent so callers can provide background agents via options (rather than manually injecting a BackgroundAgentsProvider into AIContextProviders), and updates tests + a sample accordingly.

Changes:

• Added HarnessAgentOptions.BackgroundAgents and BackgroundAgentsProviderOptions to configure background agents support.
• Updated HarnessAgent to automatically include BackgroundAgentsProvider when a non-empty background agent collection is provided.
• Updated unit tests and the Harness background-agents sample to use the new options-based configuration.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgentOptions.cs	Adds new options for configuring background agents and provider options.
dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgent.cs	Wires BackgroundAgentsProvider into default context providers when configured.
dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/HarnessAgentOptionsTests.cs	Extends options tests to cover the new properties.
dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/HarnessAgentTests.cs	Adds tests for inclusion/exclusion of BackgroundAgentsProvider.
dotnet/samples/02-agents/Harness/Harness_Step02_Research_WithBackgroundAgents/Program.cs	Switches the sample to use BackgroundAgents instead of manually adding the provider.

Comments suppressed due to low confidence (1)

dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/HarnessAgentTests.cs:1321

• This test claims to verify that multiple background agents are passed to the provider, but it only checks that a BackgroundAgentsProvider exists. Consider asserting that both agent names appear in the provider’s generated context (or otherwise verifying the provider was constructed with both agents).

    /// <summary>
    /// Verify that multiple background agents are all passed to the provider.
    /// </summary>
    [Fact]
    public void BackgroundAgentsProvider_IncludesMultipleAgents()
    {
        // Arrange
        var chatClient = new Mock<IChatClient>().Object;
        var agent1Mock = new Mock<AIAgent>();
        agent1Mock.Setup(a => a.Name).Returns("Agent1");
        var agent2Mock = new Mock<AIAgent>();
        agent2Mock.Setup(a => a.Name).Returns("Agent2");
        var options = CreateAllDisabledOptions();
        options.BackgroundAgents = [agent1Mock.Object, agent2Mock.Object];

        // Act
        var agent = new HarnessAgent(chatClient, TestMaxContextWindowTokens, TestMaxOutputTokens, options);
        var innerAgent = agent.GetService<ChatClientAgent>();

        // Assert
        Assert.NotNull(innerAgent?.AIContextProviders);
        Assert.Contains(innerAgent!.AIContextProviders!, p => p is BackgroundAgentsProvider);
    }