feat: add class name to GitHub Reporter test output

[thomhurst]

Summary

• Adds the class name to the Test column in the GitHub Reporter Details table
• Format changed from MethodName(args) to ClassName.MethodName(args)
• Makes it easier to identify which specific test is being referenced

Fixes #4323

Test plan

• Verified build compiles successfully
• Ran GitHubReporter unit tests - all 4 pass
• Verify on a real GitHub Actions run that the format looks correct

🤖 Generated with Claude Code

[thomhurst]

Summary: Adds class name to GitHub Reporter test output

Critical Issues: None found

Suggestions:

• Consider consistency with JUnitXmlWriter.cs which uses similar pattern for TestMethodIdentifierProperty.TypeName
• The null-safety fallback logic is correct and handles cases where class info may not be available

TUnit Rules Compliance:
✅ No dual-mode concerns
✅ No snapshot test updates needed
✅ No public API changes
✅ No reflection concerns
✅ No performance concerns
✅ No VSTest usage

Verdict: APPROVE - Clean change that improves test identification in GitHub Actions