Support contentMediaType for OpenAPI 3.1 schemas (parse + ContentMediaTypeIn30 rule)

[takayamaki]

Continuing the OpenAPI 3.1 work from #152.

OpenAPI 3.1 adopts JSON Schema 2020-12, which brings the contentMediaType annotation.
It records the media type of a string that carries an embedded payload.
3.0 has no keyword for this.

This PR adds parse support and version-mismatch detection.

Parse layer

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

schema.contentMediaType # => "image/png"

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

contentMediaType is an annotation: it describes the payload rather than constraining it,
so there is no runtime validation to add here.

SpecValidator rule

ContentMediaTypeIn30 reports a violation for each schema in a 3.0 document that uses contentMediaType:

OpenAPIParser.load(
  'spec.yaml',
  strict_specification_version: :warn,
)
# [ContentMediaTypeIn30] #/components/schemas/Attachment — `contentMediaType` is a 3.1 addition (from JSON Schema 2020-12); 3.0 has no equivalent

Detection inspects raw_schema key presence,
so it fires on any declared media type value.