fluentassertions/fluentassertions ? reverse-engineered prompt

Build me a .NET library that makes unit test assertions feel natural and easy to read, like numbers.Should().OnlyContain(n => n > 0) instead of lots of noisy test code.

I want it to work across common .NET test setups, including MSTest, NUnit, xUnit, and similar frameworks, and it should support older and newer .NET targets too. The main goal is to give really clear failure messages when something is wrong, so tests tell me exactly what was expected and what was actually found.

Please include a solid set of fluent extension methods for collections, strings, objects, exceptions, and other common test cases, plus enough tests to prove the behavior. If there are any public API changes or compatibility concerns, make it easy to spot those and keep the package versioning sensible.

If you need to check the latest docs or best practices for .NET testing, feel free to look them up online as you go.