Sync SonarMark with TemplateDotNetTool template updates (PRs #41, #44, #45, #46)

[Copilot]

Applies four recent upstream template changes to keep SonarMark consistent with TemplateDotNetTool.

Changes

Agent documentation (test-developer.md)

• Added Test Source Filters section documenting windows@, ubuntu@, net*.0@, dotnet*.x@ prefixes for requirements.yaml test links
• Added MSTest V4 Best Practices section with 5 antipatterns, including new Assert.StartsWith guidance (prefer over Assert.IsTrue(value.StartsWith(...)))

Main csproj

• LangVersion: 12 → latest
• GenerateDocumentationFile: True → true (consistent casing)
• Added Polyfill 9.12.0 source-only package
• Updated SonarAnalyzer.CSharp: 10.19.0 → 10.20.0
• Added explanatory comments to PrivateAssets/IncludeAssets package references

Test csproj

• GenerateDocumentationFile: True → true
• Added <Using Include="Polyfills" /> implicit using
• Updated Microsoft.NET.Test.Sdk: 18.0.1 → 18.3.0
• Updated SonarAnalyzer.CSharp: 10.19.0 → 10.20.0

Defensive null checks

• Validation.Run: added ArgumentNullException.ThrowIfNull(context)
• Context.Create (both overloads): added ArgumentNullException.ThrowIfNull(args)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code quality improvement

Pre-Submission Checklist

Before submitting this pull request, ensure you have completed the following:

Build and Test

• Code builds successfully: dotnet build --configuration Release
• All tests pass: dotnet test --configuration Release
• Code produces zero warnings

Code Quality

• Code formatting is correct: dotnet format --verify-no-changes
• New code has appropriate XML documentation comments
• Static analyzer warnings have been addressed

Linting

• Markdown linter passes: npx markdownlint-cli2 "**/*.md" "#node_modules"
• Spell checker passes: npx cspell "**/*.{md,cs}" --no-progress
• YAML linter passes: yamllint .

Testing

• Added unit tests for new functionality
• Updated existing tests if behavior changed
• All tests follow the AAA (Arrange, Act, Assert) pattern
• Test coverage is maintained or improved

Documentation

• Updated README.md (if applicable)
• Updated ARCHITECTURE.md (if applicable)
• Added code examples for new features (if applicable)
• Updated requirements.yaml (if applicable)

Additional Notes

All 76 tests pass. CodeQL reports 0 alerts. No vulnerabilities found in the new/updated dependencies.

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