Apply TemplateDotNetTool consistency updates to SarifMark

[Copilot]

Syncs SarifMark with recent TemplateDotNetTool template commits. Covers package upgrades, null safety guards, and agent documentation gaps.

Description

Project / Package Updates

• LangVersion: 12 → latest (both csproj files)
• GenerateDocumentationFile: True → true (casing consistency)
• SonarAnalyzer.CSharp: 10.19.0.132793 → 10.20.0.135146 (both csproj files)
• Microsoft.NET.Test.Sdk: 18.0.1 → 18.3.0 (test csproj)
• Added Polyfill 9.12.0 (source-only, main csproj) — enables ArgumentNullException.ThrowIfNull on all target frameworks
• Added <Using Include="Polyfills" /> implicit global using (test csproj)
• Added explanatory comments on PrivateAssets/IncludeAssets PackageReference groups

Null Guards

Added ArgumentNullException.ThrowIfNull guards in three entry points:

public static Context Create(string[] args)
{
    ArgumentNullException.ThrowIfNull(args);
    // ...
}

public void ParseArguments(string[] args)
{
    ArgumentNullException.ThrowIfNull(args);
    // ...
}

public static void Run(Context context)
{
    ArgumentNullException.ThrowIfNull(context);
    // ...
}

Agent Documentation

• test-developer.md: Added MSTest anti-pattern Add ToMarkdown method to SarifResults for markdown export #5 — prefer Assert.StartsWith over Assert.IsTrue(value.StartsWith(...))
• requirements-agent.md: Added "Test Source Filters" section documenting windows@, ubuntu@, net8.0@, dotnet9.x@, etc. prefixes
• code-quality-agent.md: Fixed markdownlint config filename (.markdownlint.json → .markdownlint-cli2.jsonc)
• AGENTS.md: Added "Test Source Filters" quick-reference section

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

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

Quality Checks

• Spell checker passes: cspell "**/*.{md,cs}"
• Markdown linter passes: markdownlint "**/*.md"
• YAML linter passes: yamllint '**/*.{yml,yaml}'
• Self-validation passes:
  dotnet run --project src/DemaConsulting.SarifMark --configuration Release --framework net10.0 --no-build -- --validate

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

Polyfill must not be added as a direct PackageReference in the test project — since the test project uses ProjectReference to the main project, adding it to both causes duplicate contentfile compilation (CS0121 ambiguous method errors on net8/net9). The implicit using alone is sufficient.

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