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.
