Sync PathHelpers.SafePathCombine null validation with TemplateDotNetTool

[Copilot]

PathHelpers.SafePathCombine was missing the ArgumentNullException.ThrowIfNull guards present in the TemplateDotNetTool reference implementation. The implementation (including all inline comments) is now identical to the template, differing only in namespace.

• PathHelpers.cs — Add // Validate inputs block with null guards before the path traversal checks:

  // Validate inputs
  ArgumentNullException.ThrowIfNull(basePath);
  ArgumentNullException.ThrowIfNull(relativePath);

• PathHelpersTests.cs — Add NullBasePath_ThrowsArgumentNullException and NullRelativePath_ThrowsArgumentNullException tests

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 unit tests pass: dotnet test --configuration Release
• Self-validation tests pass:
  dotnet run --project src/DemaConsulting.VersionMark --configuration Release --framework net10.0
--no-build -- --validate
• 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 .

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

CodeQL scan returned 0 alerts.

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