fix(test-type-safety): remove unsafe type casts and add factory helper

fix(test-type-safety): remove unsafe type casts and add factory helper

Phase 1 (Type Safety):
- Remove 16 'undefined as unknown as' casts from 8 auth test files
- Add createStationWithCertificateManager() helper to OCPP20TestUtils
- Migrate 3 certificate test files to use the new type-safe helper

Phase 2 (Enhancements):
- Add explanatory comments to 9 ESLint override directives
- Update TEST_STYLE_GUIDE.md with new factory documentation