Describes a type in an OpenAPI specification.

Types may carry constraints, expressed as a keyword list. Scalar constraints (enum, format, nullable, min_length, max_length, minimum, maximum, pattern) attach to the schema itself - or, for array types, to the array items. min_items and max_items attach to the array. Nullability is rendered according to the target OpenAPI version: nullable: true for 3.0.x, a "null" type union for 3.1.x.