Add Polyfills implicit using to test project

[Copilot]

Pull Request

Description

The test project lacked an implicit using Polyfills;, preventing test code from using Polyfill extension methods without explicit using statements in each file.

Adds <Using Include="Polyfills" /> to the test csproj. No Polyfill package reference is needed — the package is already included in the main library with PrivateAssets="All", and its compiled members flow into the test project via InternalsVisibleTo.

<!-- Implicit Usings -->
<ItemGroup>
  <Using Include="Polyfills" />
</ItemGroup>

Note: adding a duplicate Polyfill package reference to the test project causes CS0121 ambiguity errors due to the types being compiled into both assemblies.

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
• 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

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