ci: modernize GitHub Actions runners and resolve xUnit test warnings

[Zaldaryon]

Summary

Modernizes GitHub Actions workflows across the repository to use current Node.js 24-compliant actions, updates the macOS installer build runner from retired macos-13 to macos-14, and resolves all five xUnit analyzer warnings in the test suites.

Key Changes

1. Test Suite Analyzer Cleanliness:

   • Optimum.Bootstrap.Core.Tests/PrerequisiteScannerTests.cs: replaced Assert.Empty on .Where with Assert.DoesNotContain to satisfy xUnit2029.
   • Optimum.Cli.Tests/NdjsonStream.cs: replaced Assert.True(set.Contains(val)) with Assert.Contains(val, set) to satisfy xUnit2017.
   • Optimum.Installer.Tests/ScreenViewModelTests.cs: supplied TestContext.Current.CancellationToken to File.WriteAllTextAsync and Task.WaitAsync to satisfy xUnit1051.

2. Runner Modernization:

   • In release-installer.yml, transitioned the osx-x64 target runner from retired macos-13 to macos-14. .NET cross-compiles cleanly on Apple Silicon runners, avoiding indefinite queuing on retired runner images.

3. Action Version Upgrades:

   • Updated actions/checkout to @v6.
   • Updated actions/setup-dotnet to @v6.
   • Updated actions/cache to @v6.
   • Updated actions/upload-artifact to @v7.
   • Updated actions/download-artifact to @v8.
   • Eliminates all runner deprecation annotations regarding Node.js 20.

Verification

• Local test run: dotnet test Optimum.Installer.slnf -c Release (245 passed, 0 failed, 0 warnings).
• YAML syntax validation: all four workflows pass yq linting cleanly.
• Workspace configuration check: make agent-check (PASS).
• Writing voice check: zero em dashes, zero en dashes, zero spaced double hyphens.