chore: sync VHDLTest with TemplateDotNetTool template updates (Feb 2026)

[Copilot]

Applies five recent commits from TemplateDotNetTool to bring VHDLTest in sync with the template.

Description

Agent guidance (test-developer.md)

• Added Test Source Filters section documenting VHDLTest-specific filters (ghdl@, nvc@, windows@, ubuntu@, dotnetX.x@) and why removing them invalidates CI evidence
• Added MSTest V4 Best Practices section with 5 anti-patterns (catch-block assertions, IsTrue for equality/counts/prefix, non-public test classes)

Source csproj

• LangVersion: 12 → latest
• Boolean casing: PackAsTool, IncludeSymbols, GenerateDocumentationFile True → true
• SonarAnalyzer.CSharp: 10.19.0.132793 → 10.20.0.135146
• Added Polyfill 9.12.0 (source-only) with explanatory comments on all analyzer/source-only package groups

Test csproj

• GenerateDocumentationFile: True → true
• SonarAnalyzer.CSharp: 10.19.0.132793 → 10.20.0.135146
• Microsoft.NET.Test.Sdk: 18.0.1 → 18.3.0
• Added <Using Include="Polyfills" /> implicit using
• Restructured with section comments (<!-- Test Framework Dependencies -->, <!-- Code Analysis Dependencies -->, <!-- Project References -->)

C# modernization

Added ArgumentNullException.ThrowIfNull() guards to Context.Create(args) and Validation.Run(context).

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

Related Issues

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

Quality Checks

Please run the following checks before submitting:

• Spell checker passes: cspell "**/*.{md,cs}"
• Markdown linter passes: markdownlint "**/*.md"
• YAML linter passes: yamllint '**/*.{yml,yaml}'

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

No functional behavior changes — all 99 existing tests pass. The Polyfill package is source-only (PrivateAssets="all"), so it adds no runtime dependency.

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