refactor(tests): improve test isolation and remove dead code

refactor(tests): improve test isolation and remove dead code

- Fix module-level state sharing in 6 OCPP 2.0 RequestService tests
- Remove unused createMockTemplate function from StationHelpers
- Remove unused TestStationHelper, TestTimerHelper, TestEnvironmentHelper classes (206 LOC)
- Add createMockAuthCache and createMockOCPPAdapter factories
- Update TEST_STYLE_GUIDE.md with test isolation best practices
- Clean up orphaned imports after dead code removal