Align PathHelpers.SafePathCombine with TemplateDotNetTool secure implementation

[Copilot]

Pull Request

Description

PathHelpers.SafePathCombine was missing the null-argument guards present in the TemplateDotNetTool reference implementation. Without these, a null input produces an uncontrolled NullReferenceException rather than a descriptive ArgumentNullException.

• PathHelpers.cs — Added ArgumentNullException.ThrowIfNull for both basePath and relativePath at method entry. The implementation is now identical to TemplateDotNetTool apart from the namespace.

// Before: no null validation
internal static string SafePathCombine(string basePath, string relativePath)
{
    if (relativePath.Contains("..") || ...

// After: matches TemplateDotNetTool
internal static string SafePathCombine(string basePath, string relativePath)
{
    // Validate inputs
    ArgumentNullException.ThrowIfNull(basePath);
    ArgumentNullException.ThrowIfNull(relativePath);

    if (relativePath.Contains("..") || ...

• PathHelpersTests.cs — Added two tests covering null basePath and null relativePath, each asserting the correct ParamName on the thrown exception.

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

CodeQL reports 0 alerts after these changes.

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