feat: add comprehensive code coverage and test reporting to CI/CD

Add comprehensive testing infrastructure with code coverage tracking:

Just Recipes:
- test-coverage: Run tests with coverage, merge profiles, show summary
- test-junit: Run tests with JUnit XML output using gotestsum

CI/CD Enhancements:
- Generate coverage on all test runs with atomic coverage mode
- Upload coverage artifacts and test results
- Publish test summaries using test-summary/action v2.4
- Add coverage-report job for PR comments with fgrosse/go-coverage-report
- Continue uploading to Codecov for historical tracking

Configuration:
- Add coverage/, coverage.out, test-results/ to .gitignore
- Update test job to use test-junit recipe for comprehensive reporting

Documentation:
- Add Testing and Code Coverage section to morphir-go-preview.md
- Document local testing commands and coverage workflows
- Explain CI/CD integration and PR coverage comments
- Include emoji indicators for coverage changes

Benefits:
- Track code coverage trends over time
- See coverage impact directly in PR reviews
- Better visibility into test results with GitHub Actions summaries
- Identify untested code paths
- Automated test result reports

Relates to morphir-rnt

Author: DamianReeves