Fix CS8632 nullable annotation warning

- Add #nullable enable directive to JobsControllerTests.cs
- Resolves CS8632 warning about nullable reference type annotations
- All 108 JobManager tests still passing
- Build succeeds with no CS8632 warnings

Co-authored-by: devstress <30769729+devstress@users.noreply.github.com>

Author: Copilot

FlinkDotNet/FlinkDotNet.JobManager.Tests/JobsControllerTests.cs

@@ -2,6 +2,8 @@
 // Licensed under the Apache License, Version 2.0.
 // See LICENSE file in the project root for full license information.
 
+#nullable enable
+
 using FlinkDotNet.JobManager.Controllers;
 using FlinkDotNet.JobManager.Interfaces;
 using FlinkDotNet.JobManager.Models;