Fix post type schema

[priethor]

What

Fixes post type schema validation and adds test coverage for edge cases.

Why

• The schema was too restrictive for menu_position (should accept null, empty string, or integer)
• The taxonomies field validation was changed from oneOf to anyOf for more flexible validation, as an empty string could match both
• Lacked test coverage for edge cases with empty/null values

Testing Instructions

1. Run composer test:php to verify all post type schema tests pass
2. Confirm 27 tests pass with 60 assertions