Add empty-team validation to MagenticWorkflowBuilder.Build() and E2E test

Agent-Logs-Url: https://github.com/microsoft/agent-framework/sessions/e490fdf7-f107-4fde-ba1f-efdfd9a729c6

Co-authored-by: lokitoth <6936551+lokitoth@users.noreply.github.com>

dotnet/src/Microsoft.Agents.AI.Workflows/MagenticWorkflowBuilder.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Threading.Tasks;
@@ -140,7 +141,15 @@ public class MagenticWorkflowBuilder(AIAgent managerAgent)
     }
 
     /// <inheritdoc cref="WorkflowBuilder.Build"/>
-    public Workflow Build() => this.ReduceToWorkflowBuilder().Build();
+    public Workflow Build()
+    {
+        if (this._team.Count == 0)
+        {
+            throw new InvalidOperationException("At least one participant must be added before building a Magentic workflow.");
+        }
+
+        return this.ReduceToWorkflowBuilder().Build();
+    }
 
     private TaskLimits Limits => new(
         MaxRoundCount: this._maxRounds,

dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/MagenticOrchestrationTests.cs

@@ -1165,6 +1165,24 @@ public class MagenticOrchestrationTests
         fourthResult.Result![0].Text.Should().Contain("Completed after multiple revisions");
     }
 
+    [Fact]
+    public void Empty_Team_Build_Throws()
+    {
+        // Arrange: No participants added to the builder.
+        TestReplayAgent manager = new(
+            [CreatePlanResponse("Facts"), CreatePlanResponse("Plan")],
+            name: "Manager");
+
+        MagenticWorkflowBuilder builder = new MagenticWorkflowBuilder(manager)
+            // No .AddParticipants() — empty team
+            .RequirePlanSignoff(false);
+
+        // Act & Assert: Build() should throw because the team is empty.
+        Action buildAction = () => builder.Build();
+        buildAction.Should().Throw<InvalidOperationException>()
+            .WithMessage("*participant*");
+    }
+
     #region Helper Methods
 
     private sealed record WorkflowRunResult(