[josephaxisa]

Fixes #304

In OpenAPI 3.0 (§4.7.24 Reference Object), a Reference Object ({"$ref": "..."}) cannot be extended and any sibling keywords next to $ref have to be ignored (matching Draft 4 / json-reference behavior). OpenAPI 3.1 and 3.2 switched to JSON Schema Draft 2020-12, where $ref siblings are evaluated alongside the referenced schema.

Because jsonschema.validators.create() defaults applicable_validators to evaluating every keyword on the schema dict (whereas Draft4Validator explicitly passes applicable_validators=_legacy_keywords.ignore_ref_siblings), _build_oas30_validator() was evaluating sibling keywords next to $ref.

In practice, OAS 3.0 specs and code-first generators often leave sibling keywords like nullable, readOnly, writeOnly, type, or minLength on $ref properties. When OAS30Validator (or downstream callers like openapi-core) validates against those schemas, two things go wrong:

• Valid payloads get rejected when a sibling keyword (nullable: false, minLength, type, required) conflicts with or narrows the referenced target schema.
• Required properties get silently skipped when a $ref property has a sibling readOnly: true or writeOnly: true, because required, read_required, and write_required in _keywords.py inspect prop_schema.get("readOnly") / prop_schema.get("writeOnly") directly on the parent's properties dict before $ref resolution.

What changed

• Pass applicable_validators=_legacy_keywords.ignore_ref_siblings in _build_oas30_validator() (openapi_schema_validator/validators.py). This is inherited via extend() by OAS30Validator, OAS30StrictValidator, OAS30ReadValidator, OAS30WriteValidator, and build_enforce_properties_required_validator(OAS30*), while leaving OAS31Validator and OAS32Validator untouched.
• Check if prop_schema and "$ref" not in prop_schema: in required, read_required, and write_required (openapi_schema_validator/_keywords.py) so sibling readOnly/writeOnly flags on a $ref property don't suppress missing-required errors in OAS 3.0.
• Added integration and unit tests covering OAS30Validator, OAS30StrictValidator, OAS30ReadValidator, OAS30WriteValidator, build_enforce_properties_required_validator, OAS31Validator, OAS32Validator, and validate().