[fix] Fix OpenAPI validation errors for CI compliance

- Convert all nullable: true to OpenAPI 3.1 format using type: [type, 'null']
- Fix invalid array schema definition in ManagerMappings using oneOf
- Add default security: [] configuration to satisfy security-defined rule
- All 41 validation errors resolved, spec now passes with 0 errors
- 141 warnings remain (mostly missing operationId and example validation)

Author: christian-byrne