Migrate documentation samples to NUnit4

[Romfos]

Changes:

• Migrate documentation samples to NUnit4
• Update readme
• Update acknowledgements
• Remove NUnit.Framework.Legacy from tests
• Enable NRE for ProtectedExtensions.cs

[Copilot]

Pull Request Overview

This PR migrates all documentation samples and tests to NUnit 4 syntax, removes legacy NUnit support, and updates related project and documentation files.

• Upgraded NUnit package to version 4
• Removed NUnit.Framework.Legacy imports and replaced ClassicAssert calls with Assert.That
• Updated documentation examples and readme/acknowledgements to reflect new syntax and tooling

Reviewed Changes

Copilot reviewed 44 out of 44 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/NSubstitute.Documentation.Tests/NSubstitute.Documentation.Tests.csproj	Updated NUnit package reference from 3.x to 4.x
tests/NSubstitute.Documentation.Tests.Generator/DocumentationTestsGenerator.cs	Removed obsolete NSubstitute.Compatibility import
src/NSubstitute/Extensions/ProtectedExtensions.cs	Enabled null-return (object?) and NRE for protected extensions
docs/index.md	Converted example Assert.AreEqual to Assert.That syntax
README.md	Streamlined build instructions with updated link
acknowledgements.md	Marked deprecated tools and updated usage notes

Comments suppressed due to low confidence (2)

README.md:131

• [nitpick] This line has lost context and proper markdown formatting. Consider restoring build instructions and formatting the link as [Release procedure](https://github.com/nsubstitute/NSubstitute/wiki/Release-procedure) for clarity.

Release-procedure https://github.com/nsubstitute/NSubstitute/wiki/Release-procedure

docs/help/setting-out-and-ref-arguments/index.md:30

• The actual and expected parameters are reversed. It should read Assert.That(value, Is.EqualTo("world!")); to follow the NUnit Assert.That(actual, constraint) pattern.

Assert.That("world!", Is.EqualTo(value));