add: test cases for automatic zorder assignment in boxplot layering

- Add comprehensive test coverage for zorder functionality
- Test both scenarios: with and without automatic zorder assignment
- Includes visual regression tests with SVG snapshots
- Verifies boxplot renders correctly on top of confidence intervals
- Addresses issue where box plots rendered behind other traces

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: huangzhibo