Support prefixItems for OpenAPI 3.1 schemas (parse + positional tuple validation + PrefixItemsIn30 rule)

[takayamaki]

Continuing the OpenAPI 3.1 work from #152.

OpenAPI 3.1 adopts the JSON Schema 2020-12 prefixItems keyword,
which validates the leading elements of an array positionally (tuple typing).
3.0 has no equivalent — it only has items, which applies uniformly to every element.

This PR adds parse support, runtime validation, and version-mismatch detection.

Parse layer

prefixItems is parsed as a list of Schema objects and exposed as an accessor:

schema.prefix_items # => [#<Schema type="string">, #<Schema type="integer">]

The Ruby accessor is snake_case with schema_key: :prefixItems,
matching how all_of / any_of / additional_properties are already declared.
Following the permissive-parse strategy agreed in #152,
the parse layer accepts prefixItems regardless of the declared OpenAPI version.

Runtime validation

ArrayValidator dispatches per position:
element i is validated against prefixItems[i] when one exists,
and falls back to items otherwise.
This is how JSON Schema 2020-12 layers the two keywords.

# schema: { type: array, prefixItems: [{type: string}, {type: integer}], items: {type: boolean} }
validate(['a', 1])              # => ok
validate(['a', 'x'])            # => ValidateError (position 1 must be an integer)
validate(['a', 1, true, false]) # => ok (extras fall through to items)
validate(['a', 1, 'x'])         # => ValidateError (extras must be booleans)

A schema with prefixItems but no items leaves the trailing elements unvalidated,
which matches JSON Schema's default of an absent items being unconstrained.

SpecValidator rule

PrefixItemsIn30 reports a violation for each schema in a 3.0 document that uses prefixItems:

OpenAPIParser.load(
  'spec.yaml',
  strict_specification_version: :warn,
)
# [PrefixItemsIn30] #/components/schemas/Tuple — `prefixItems` is a 3.1 addition (from JSON Schema 2020-12); 3.0 has no equivalent