OAS30Validator evaluates sibling keywords alongside $ref instead of ignoring them per OpenAPI 3.0 §4.7.24 #304

Description

In OpenAPI 3.0.x (§4.7.24 Reference Object and JSON Schema Draft 4 / json-reference), a Reference Object ({"$ref": "..."}) cannot be extended with additional properties and any sibling keywords added alongside $ref SHALL be ignored:

This object cannot be extended with additional properties and any properties added SHALL be ignored.

Currently, _build_oas30_validator() calls jsonschema.validators.create(...) without passing applicable_validators=_legacy_keywords.ignore_ref_siblings (unlike jsonschema.Draft4Validator, which sets applicable_validators=_legacy_keywords.ignore_ref_siblings because create() defaults to evaluating all items as in Draft 2019-09 / 2020-12). In addition, required, read_required, and write_required in _keywords.py inspect prop_schema.get("readOnly") / prop_schema.get("writeOnly") directly on child property schemas even when prop_schema contains "$ref".

As a result, OAS30Validator (and OAS30ReadValidator, OAS30WriteValidator, OAS30StrictValidator) evaluates sibling keywords such as type, nullable, minLength, readOnly, writeOnly, and required when they appear alongside $ref, rather than ignoring them as required by OAS 3.0 (OAS31Validator and OAS32Validator should continue evaluating $ref siblings per JSON Schema Draft 2020-12).

Minimal Reproducer

from openapi_schema_validator import OAS30Validator, validate

schema = {
    "type": "object",
    "required": ["id"],
    "properties": {
        "id": {"type": "integer"},
        "name": {
            "$ref": "#/components/schemas/NullableName",
            "nullable": False,
            "minLength": 10,
        },
    },
    "components": {
        "schemas": {
            "NullableName": {
                "type": "string",
                "nullable": True,
                "minLength": 2,
            }
        }
    },
}

# Raises ValidationError ("None for not nullable" / "'abc' is too short")
# even though `nullable: False` and `minLength: 10` are $ref siblings in OAS 3.0:
validate({"id": 1, "name": None}, schema, cls=OAS30Validator)