OpenAPI schema supporting the M21 emitted vocabulary:

• Primitive types: {@code string}, {@code number}, {@code integer}, {@code boolean}, {@code null}
• Arrays: {@code type: 'array'} with {@code items}
• Objects: {@code type: 'object'} with {@code properties}/{@code required}/{@code additionalProperties}
• Enums: {@code enum} array of literals
• Const: {@code const} single literal
• References: {@code $ref}
• Unions: {@code anyOf} (used for nullable schemas)
• Intersections: {@code allOf}
• {@code oneOf} (union, mapped like {@code anyOf})
• Empty schema → {@code unknown}