[scala-client]fix missing type in StringToArrayModelMapping

Created by: bgong-mdsol

This PR fixes the issue #11721

Here is the openapi spec that can be used to reproduce the issue and verify the fix.

openapi: 3.0.0
info:
  title: stringToModelMapping
  version: "1.0.0"
paths:
  /:
    get:
      summary: Empty Route
      responses:
        "200":
          description: Good Request.
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/FreeformObjectTypes'
        "400":
          description: Bad Request.
        "403":
          description: Forbidden. Access denied.
components:
  schemas:
    SimpleModel:
      type: object
      properties:
        name:
          type: string
    ArrayModel:
      type: array
      items:
        $ref: '#/components/schemas/SimpleModel'
    FreeformObjectTypes:
      type: object
      properties:
        stringToModelMapping:
          type: object
          additionalProperties:
            $ref: '#/components/schemas/SimpleModel'
        stringToArrayModelMapping:
          type: object
          additionalProperties:
            $ref: '#/components/schemas/ArrayModel'

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