Update NUnit to v4

[Trenly]

📖 Description

Upgrade the AppInstallerCLIE2ETests suite to NUnit 4 and modernize the supporting test code to the NUnit 4 constraint model. This lands on the latest available NUnit 4.x release in NuGet (4.6.1), while leaving NUnit3TestAdapter unchanged.

The update converts the E2E suite off NUnit 3 classic assertions, fixes the delegate-based assertion overloads required by NUnit 4, and carries the package version change through the affected test projects that use centralized package management.

I opted for full migration instead of the simpler Directory.Build.props change as I expect this will be a one-time migration and I generally believe that using the newer format will be more performant in the future (plus I had the extra tokens to burn, so why not)

🔗 References

• Split from Update NuGet Packages #6258
• NUnit 4 migration guide: https://docs.nunit.org/articles/nunit/release-notes/Nunit4.0-MigrationGuide.html

🔍 Validation

• Search-based verification confirmed the old NUnit 3 classic assertions were removed from src/AppInstallerCLIE2ETests.

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated Release Notes (if applicable)
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow