Support const for OpenAPI 3.1 schemas (parse + runtime + ConstIn30 rule)

[takayamaki]

Continuing the OpenAPI 3.1 work from #152.

OpenAPI 3.1 adopts the JSON Schema 2020-12 const keyword, which pins a value to exactly one constant.
3.0 does not recognize it (the closest 3.0 idiom is a single-element enum).

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

Parse layer

const is parsed as a plain value and exposed as an accessor on Schema:

schema.const # => "fixed-value"

Following the permissive-parse strategy agreed in #152,
the parse layer accepts const regardless of the declared OpenAPI version.

Runtime validation

const is checked before type dispatch in validate_schema,
so it applies uniformly across primitives, arrays, and objects:

# schema: { const: "USD" }
request_operation.validate_request_body('application/json', { 'currency' => 'JPY' })
# => OpenAPIParser::ValidateError: "JPY" const "USD" invalid

Detection inspects raw_schema key presence,
so an intentional const: null still constrains the value to null.

SpecValidator rule

ConstIn30 reports a violation for each schema in a 3.0 document that uses const:

OpenAPIParser.load(
  'spec.yaml',
  strict_specification_version: :warn,
)
# [ConstIn30] #/components/schemas/Currency — `const` is a 3.1 addition (from JSON Schema 2020-12); 3.0 has no equivalent

[geemus]

lgtm, thanks!