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/DocumentOperator.Tests.csproj

@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
+    <PackageReference Include="xunit" Version="2.5.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+</Project>

DocumentOperator.Tests/UnitTest1.cs

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