Add unit tests for call stack depth calculations and persistence

- Add `WorkflowExecutionContextTests` to verify correct calculation of call stack depth during activity execution.
- Add `WorkflowStateExtractorTests` to ensure call stack depth is preserved during state extraction and application.

Author: sfmskywalker

test/unit/Elsa.Workflows.Core.UnitTests/Contexts/WorkflowExecutionContextTests.cs

@@ -0,0 +1,45 @@
+using Elsa.Common;
+using Elsa.Testing.Shared;
+using Elsa.Workflows.Activities;
+using Elsa.Workflows.Options;
+using Microsoft.Extensions.DependencyInjection;
+using Xunit;
+
+namespace Elsa.Workflows.Core.UnitTests.Contexts;
+
+public class WorkflowExecutionContextTests
+{
+    [Fact]
+    public async Task CreateActivityExecutionContextAsync_CalculatesCallStackDepth_Correctly()
+    {
+        // Arrange
+        var activityA = new WriteLine("A");
+        var activityB = new WriteLine("B");
+        var activityC = new WriteLine("C");
+        var root = new Sequence { Activities = { activityA, activityB, activityC } };
+        var fixture = new ActivityTestFixture(root);
+        var contextRoot = await fixture.BuildAsync();
+        var workflowExecutionContext = contextRoot.WorkflowExecutionContext;
+
+        // Act
+        // Level 0 (already created by fixture.BuildAsync for the root activity, but let's create explicitly for clarity)
+        var contextA = await workflowExecutionContext.CreateActivityExecutionContextAsync(activityA);
+        
+        // Level 1: B scheduled by A
+        var contextB = await workflowExecutionContext.CreateActivityExecutionContextAsync(activityB, new ActivityInvocationOptions
+        {
+            SchedulingActivityExecutionId = contextA.Id
+        });
+        
+        // Level 2: C scheduled by B
+        var contextC = await workflowExecutionContext.CreateActivityExecutionContextAsync(activityC, new ActivityInvocationOptions
+        {
+            SchedulingActivityExecutionId = contextB.Id
+        });
+
+        // Assert
+        Assert.Equal(0, contextA.CallStackDepth);
+        Assert.Equal(1, contextB.CallStackDepth);
+        Assert.Equal(2, contextC.CallStackDepth);
+    }
+}

test/unit/Elsa.Workflows.Core.UnitTests/Services/WorkflowStateExtractorTests.cs

@@ -0,0 +1,46 @@
+using Elsa.Common;
+using Elsa.Testing.Shared;
+using Elsa.Workflows.Activities;
+using Elsa.Workflows.Services;
+using Elsa.Workflows.State;
+using Microsoft.Extensions.DependencyInjection;
+using Xunit;
+
+namespace Elsa.Workflows.Core.UnitTests.Services;
+
+public class WorkflowStateExtractorTests
+{
+    [Fact]
+    public async Task Extract_And_Apply_PreservesCallStackDepth()
+    {
+        // Arrange
+        var root = new WriteLine("root");
+        var fixture = new ActivityTestFixture(root);
+        var contextRoot = await fixture.BuildAsync();
+        var workflowExecutionContext = contextRoot.WorkflowExecutionContext;
+        
+        var contextA = await workflowExecutionContext.CreateActivityExecutionContextAsync(root);
+        contextA.CallStackDepth = 10; // Manually set for testing persistence
+        workflowExecutionContext.AddActivityExecutionContext(contextA);
+
+        var extractor = workflowExecutionContext.GetRequiredService<IWorkflowStateExtractor>();
+
+        // Act
+        var state = extractor.Extract(workflowExecutionContext);
+        
+        // Create a new context to apply the state to
+        var newWorkflowExecutionContext = await WorkflowExecutionContext.CreateAsync(
+            workflowExecutionContext.ServiceProvider,
+            workflowExecutionContext.WorkflowGraph,
+            state.Id,
+            CancellationToken.None
+        );
+        
+        await extractor.ApplyAsync(newWorkflowExecutionContext, state);
+
+        // Assert
+        var restoredContextA = newWorkflowExecutionContext.ActivityExecutionContexts.FirstOrDefault(x => x.Id == contextA.Id);
+        Assert.NotNull(restoredContextA);
+        Assert.Equal(10, restoredContextA.CallStackDepth);
+    }
+}