Add unit tests for NotFoundActivity to validate exception behavior (#7115)

- Introduced `NotFoundActivityTests` to cover scenarios of missing activity types and versions.
- Added tests to assert `ActivityNotFoundException` handling and inclusion of type name and version in exception messages.

test/unit/Elsa.Activities.UnitTests/Primitives/NotFoundActivityTests.cs

@@ -0,0 +1,69 @@
+using Elsa.Testing.Shared;
+using Elsa.Workflows;
+using Elsa.Workflows.Exceptions;
+
+namespace Elsa.Activities.UnitTests.Primitives;
+
+public class NotFoundActivityTests
+{
+    [Fact(DisplayName = "NotFoundActivity throws ActivityNotFoundException")]
+    public async Task Should_Throw_ActivityNotFoundException()
+    {
+        // Arrange
+        const string typeName = "MyMissingActivity";
+
+        // Act & Assert
+        var exception = await ExecuteAndAssertExceptionAsync(typeName);
+        Assert.Equal(typeName, exception.MissingTypeName);
+    }
+
+    [Fact(DisplayName = "NotFoundActivity includes type name in exception")]
+    public async Task Should_Include_TypeName_In_Exception()
+    {
+        // Arrange
+        const string typeName = "Custom.Namespace.MyActivity";
+
+        // Act & Assert
+        var exception = await ExecuteAndAssertExceptionAsync(typeName);
+        Assert.Contains(typeName, exception.Message);
+    }
+
+    [Fact(DisplayName = "NotFoundActivity includes version in exception")]
+    public async Task Should_Include_Version_In_Exception()
+    {
+        // Arrange
+        const string typeName = "MyMissingActivity";
+        const int version = 2;
+
+        // Act & Assert
+        var exception = await ExecuteAndAssertExceptionAsync(typeName, version);
+        Assert.Equal(version, exception.MissingTypeVersion);
+        Assert.Contains(version.ToString(), exception.Message);
+    }
+
+    [Theory(DisplayName = "NotFoundActivity preserves various type names")]
+    [InlineData("SimpleActivity")]
+    [InlineData("Namespace.Activity")]
+    [InlineData("My.Custom.Namespace.ComplexActivity")]
+    [InlineData("Activity123")]
+    public async Task Should_Preserve_Various_TypeNames(string typeName)
+    {
+        // Act & Assert
+        var exception = await ExecuteAndAssertExceptionAsync(typeName);
+        Assert.Equal(typeName, exception.MissingTypeName);
+    }
+
+    private static async Task<ActivityNotFoundException> ExecuteAndAssertExceptionAsync(string typeName, int version = 0)
+    {
+        var notFoundActivity = new NotFoundActivity(typeName, null, null)
+        {
+            MissingTypeVersion = version
+        };
+        return await Assert.ThrowsAsync<ActivityNotFoundException>(() => ExecuteAsync(notFoundActivity));
+    }
+
+    private static async Task<ActivityExecutionContext> ExecuteAsync(IActivity activity)
+    {
+        return await new ActivityTestFixture(activity).ExecuteAsync();
+    }
+}