feat(dotnet): add JUnit XML output, upgrade to .NET 8, and refactor to Calculator classes

[Konboi]

Why

The dotnet example needed several improvements:

• It only outputted NUnit XML format, but JUnit XML format was also needed for broader compatibility with test reporting tools.
• The project was targeting .NET 7 which reached end-of-life in May 2024.
• The sample code was a simple Example class that didn't clearly demonstrate how test reports look across multiple test classes.

What

• Add JUnit XML output: Added JunitXml.TestLogger package and updated dotnet test command to output both test-result.xml (NUnit format) and junit-result.xml (JUnit format).
• Upgrade to .NET 8 LTS: Updated TargetFramework to net8.0 and SDK version to 8.0.x in the CI workflow. Also updated all package versions to be compatible with .NET 8.
• Refactor to Calculator classes: Replaced Example class with a proper Calculator structure:
  • Individual operation classes: AddCalculator, SubtractCalculator, MultiplyCalculator, DivideCalculator, PowerCalculator
  • A Calculator facade class that composes all operation classes
  • Separate test class per operation class to demonstrate how multiple test classes appear in the XML reports
• Add intentional failing tests: Added TestAddFailing and TestDivideFailing with a comment indicating they are intentionally failing, to verify that failure reporting works correctly in both XML formats.

Please Review Here (Optional)

• Package versions for NunitXml.TestLogger was bumped to 8.0.0 to fix a get_InputSanitizer interface compatibility error with the newer test platform (17.8.0).

[Konboi]

CI failed due to this intentional failed test

[Konboi]

dotnet 7 is already EOL