[BUG] Handled edge cases where a map can have predefined keys

Description

this schema is an edge case of a dynamic map with some keys defined beforehand, but it doesn't get handled very well in the generator

"properties": {
          "extraProperties": {
            "type": "object",
            "properties": {
              "A": {
                "$ref": "#/components/schemas/AType"
              },
              "B": {
                "$ref": "#/components/schemas/BType"
              },
              "C": {
                "$ref": "#/components/schemas/CType"
              }
            },
            "additionalProperties": { },
            "nullable": true,
            "readOnly": true
          },
          "id": {
            "type": "string",
            "format": "uuid"
          },
}

the generator defines a new model for the extra properties, but sets its isModel: false , isContainer: false, isMap: false
I think it should assign all of them to true

openapi-generator version

6.0.0

cc @wing328 since it might be a core issue that affects all generators
@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

Start by reproducing the supplied OpenAPI schema in the generator and trace how the model for extraProperties is created. Verify that the generated model is classified as a model, container, and map, and add or update coverage for this schema if the relevant test location can be identified.