Add DocumentOperator.Tests project for unit testing

Added a new test project, `DocumentOperator.Tests`, to the solution targeting .NET 8.0. Configured the project as a non-packable test project with support for nullable reference types and implicit usings. Included dependencies for `xunit`, `Microsoft.NET.Test.Sdk`, and `coverlet.collector` for testing and code coverage.

Added a placeholder test class, `UnitTest1`, with a single test method, `Test1`, marked with the `[Fact]` attribute.

DocumentOperator.Tests/UnitTest1.cs

@@ -0,0 +1,11 @@
+namespace DocumentOperator.Tests
+{
+    public class UnitTest1
+    {
+        [Fact]
+        public void Test1()
+        {
+
+        }
+    }
+}