`default` property sibling to a `$ref` is ignored

Describe the bug
default property sibling to a $ref is ignored. OAS 3.1.X relies on JSON Schema 2020-12 which does not include the requirement that sibling properties to $ref should be ignored.

OpenAPI Spec File

{
  "openapi": "3.1.0",
  "info": {
    "title": "title",
    "summary": "summary",
    "description": "description",
    "version": "0.0.1"
  },
  "paths": {
    "/": {
      "get": {
        "parameters": [
          {
            "name": "default_lost1",
            "required": false,
            "schema": {
              "$ref": "#/components/schemas/String",
              "default": "lost"
            }
          },
          {
            "name": "default_lost2",
            "required": false,
            "schema": {
              "$ref": "#/components/schemas/DefaultString"
            }
          },
          {
            "name": "default_lost3",
            "schema": {
              "allOf": [
                {
                  "$ref": "#/components/schemas/DefaultString"
                }
              ]
            }
          }, 
          {
            "name": "default_kept",
            "schema": {
              "allOf": [
                {
                  "$ref": "#/components/schemas/String"
                }
              ],
              "default": "kept"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "Successful Response",
            "content": {}
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "String": {
        "type": "string"
      },
      "DefaultString": {
        "type": "string",
        "default": "component_default"
      }
    }
  }
}

Desktop (please complete the following information):

• OS: macOS 26.4.1
• Python Version: 3.11.13
• openapi-python-client version: 0.28.3

Additional context
The previous schema generates code like this:

def _get_kwargs(
    *,
    body: Object,
    default_lost1: str | Unset = UNSET,
    default_lost2: str | Unset = UNSET,
    default_lost3: str | Unset = UNSET,
    default_kept: str | Unset = 'kept',

) -> dict[str, Any]:

I would expect all defaults to be kept.

If anything else is needed I will be more than happy to provide it. Thank you for your time!

首先使用报告中的 OpenAPI 3.1 规范重现该问题，并检查生成的 _get_kwargs 输出。跟踪直接 $ref sibling、被引用的 schema 以及 allOf wrapper 的 schema 默认值处理方式。生成的客户端保留全部四个默认值（包括 component_default）即表示完成。