openapi_3_1 SwaggerUI does not display custom extensions

[originalrusyn]

Describe the bug

When configuring springdoc.api-docs.version=openapi_3_1, SwaggerUI does not display custom extensions defined in the OpenAPI specification. The extensions are visible in the OpenAPI JSON or YAML output but do not appear in the SwaggerUI interface.

To Reproduce Steps to reproduce the behavior:

• What version of spring-boot you are using ?

spring-boot-*:3.3.2

• What modules and versions of springdoc-openapi are you using?

springdoc-openapi-starter-common:2.6.0 springdoc-openapi-starter-webmvc-api:2.6.0 springdoc-openapi-starter-webmvc-ui:2.6.0

• What is the actual and the expected result using OpenAPI Description (yml or json)?

openapi: 3.1.0
info:
  title: API
servers:
  - description: Generated server url
    url: http://localhost:8080/api
paths:
    get:
      description: Fetch Obj
      operationId: fetchObj
      parameters:
        - description: id of the obj
          in: path
          name: objId
          required: true
          schema:
            type:
              - integer
            format: int64
            example: null
      responses:
        "200":
          content:
            '*/*':
              schema:
                $ref: "#/components/schemas/ObjV1"
                example: null
          description: obj
components:
  schemas:
    ObjV1:
      description: obj
      example: null
      properties:
        description:
          type:
            - string
          description: description of the obj
          example: null
          maxLength: 255
          minLength: 0
        id:
          type:
            - integer
          format: int64
          description: id of the obj
          example: null
          readOnly: true
        name:
          type:
            - string
          format: uri
          description: desc
          example: null
          x-custom-extension:
            maxLength: 255

• Provide with a sample code (HelloController) or Test that reproduces the problem

/**
     * Obj
     *
     * @param id id of the obj
     * @param name 
     */
    public record ObjV1(
        @Schema(accessMode = Schema.AccessMode.READ_ONLY) long id,
        @Schema( extensions = {@Extension(name = "x-custom-extension", properties = {@ExtensionProperty(name = "maxLength", value = "255", parseValue = true)})}) @Nullable String name
    ) {
    }

Expected behavior

Custom extensions defined in the OpenAPI specification should be visible in SwaggerUI. For instance, x-custom-extension should appear under the relevant schema properties in the SwaggerUI interface.

Additional context It works as expected for springdoc.api-docs.version=openapi_3_0