Detect nullable in OpenAPI 3.1 documents (NullableDeprecation rule)

[takayamaki]

ref: #152

Continuing the OpenAPI 3.1 work from #192.

This PR adds a new SpecValidator rule:

• OpenAPIParser::SpecValidator::Rules::NullableDeprecation

Design

In 3.0, nullable: true marks a schema as accepting null.
3.1 removed the keyword entirely in favor of type: [..., "null"].

The rule flags the mere presence of nullable on a 3.1 document — true and false are both violations, since the field itself is gone.
This matches how OAS treats removed keywords: it is not about the value, it is about the keyword being there at all.

# 3.1 document
schema:
  type: string
  nullable: false   # flagged — keyword removed in 3.1, even though false is the safe value

Implementation details

Detection inspects raw_schema directly

schema.nullable returns false when the key is absent and when it is explicitly set to false. The rule needs to distinguish those two states,
so it checks schema.raw_schema.key?('nullable') instead of the accessor:

each_schema(root) do |schema|
  next unless schema.raw_schema.is_a?(Hash) && schema.raw_schema.key?('nullable')
  violations << violation(path: schema.object_reference, message: ...)
end

Wiring

One line added to the rules array in SpecValidator, plus the rule file under spec_validator/rules/. No changes to the parse layer or runtime
— parsing continues to accept nullable on either version; only the validator reports it.

Tests

• Unit specs cover: 3.0 with nullable: true / false (clean), 3.1 with nullable: true / false (both flagged), 3.1 without nullable
  (clean), :unknown version (skipped).
• Integration spec exercises the warn/raise dispatch path with openapi_3_1/nullable_30.yaml and nullable_31.yaml fixtures.