.NET: Add background agents support to HarnessAgent (#5977)

* Add background agents support to HarnessAgent

* Add unit tests

* Address PR comments

dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgent.cs

@@ -4,6 +4,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
+using System.Linq;
 using Microsoft.Agents.AI.Compaction;
 using Microsoft.Extensions.AI;
 using Microsoft.Shared.DiagnosticIds;
@@ -249,6 +250,15 @@ public sealed class HarnessAgent : DelegatingAIAgent
             providers.Add(skillsProvider);
         }
 
+        if (options?.BackgroundAgents is IEnumerable<AIAgent> backgroundAgents)
+        {
+            var materializedAgents = backgroundAgents.ToList();
+            if (materializedAgents.Count > 0)
+            {
+                providers.Add(new BackgroundAgentsProvider(materializedAgents, options.BackgroundAgentsProviderOptions));
+            }
+        }
+
         if (options?.AIContextProviders is IEnumerable<AIContextProvider> userProviders)
         {
             providers.AddRange(userProviders);

dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgentOptions.cs

@@ -218,4 +218,26 @@ public sealed class HarnessAgentOptions
     /// This property is ignored when <see cref="DisableOpenTelemetry"/> is <see langword="true"/>.
     /// </remarks>
     public string? OpenTelemetrySourceName { get; set; }
+
+    /// <summary>
+    /// Gets or sets the collection of background agents available for delegation via <see cref="BackgroundAgentsProvider"/>.
+    /// </summary>
+    /// <remarks>
+    /// When non-null and non-empty, a <see cref="BackgroundAgentsProvider"/> is automatically included in the
+    /// agent's context providers, enabling the agent to start, monitor, and retrieve results from background tasks.
+    /// When <see langword="null"/> or empty, no <see cref="BackgroundAgentsProvider"/> is configured.
+    /// Each agent in the collection must have a non-empty <see cref="AIAgent.Name"/> and names must be unique
+    /// (case-insensitive). If these requirements are not met, <see cref="BackgroundAgentsProvider"/> will throw
+    /// an <see cref="System.ArgumentException"/> during construction.
+    /// </remarks>
+    public IEnumerable<AIAgent>? BackgroundAgents { get; set; }
+
+    /// <summary>
+    /// Gets or sets optional configuration for the <see cref="BackgroundAgentsProvider"/>.
+    /// </summary>
+    /// <remarks>
+    /// Use this to customize instructions or agent list formatting for the background agents feature.
+    /// This property is ignored when <see cref="BackgroundAgents"/> is <see langword="null"/> or empty.
+    /// </remarks>
+    public BackgroundAgentsProviderOptions? BackgroundAgentsProviderOptions { get; set; }
 }