Support root-level jsonSchemaDialect for OpenAPI 3.1 documents (parse + JsonSchemaDialectIn30 rule)

[takayamaki]

Continuing the OpenAPI 3.1 work from #152.

OpenAPI 3.1 adds a root-level jsonSchemaDialect field:
a URI declaring the default JSON Schema dialect for schemas in the document.
3.0 has no equivalent field.

This PR adds parse support and version-mismatch detection.

Parse layer

jsonSchemaDialect is parsed as a plain string value,
exposed as an accessor on the root object:

root = OpenAPIParser.load('spec_3_1.yaml')
root.json_schema_dialect # => "https://spec.openapis.org/oas/3.1/dialect/base"

Following the permissive-parse strategy agreed in #152,
the parse layer accepts jsonSchemaDialect regardless of the declared OpenAPI version.
Dialect-aware schema interpretation is out of scope for this PR;
the value is exposed for callers to inspect.

SpecValidator rule

JsonSchemaDialectIn30 reports a violation when a 3.0 document declares root-level jsonSchemaDialect:

OpenAPIParser.load(
  'spec.yaml',
  strict_specification_version: :warn,
)
# [JsonSchemaDialectIn30] #/jsonSchemaDialect — `jsonSchemaDialect` is a 3.1 root-level addition; 3.0 documents have no such field

[geemus]

lgtm, thanks!