[BUG] [typescript] `anyOf` properties produce malformed import statements

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When schemas specifying an anyOf property as an input, the generator produces import statements which are not valid typescript. For example if a return value is typed as being any of string or number:

"anyOf": [{ "type": "string" }, { "type": "number" }]

The following import statements are produced:

import { String | number } from '../models/string | number';

openapi-generator version

6.0.0-beta
5.4.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.2",
  "info": {
    "version": "0.0.1",
    "title": "anyOf bug reproduction"
  },
  "paths": {
    "/example-object": {
      "get": {
        "tags": ["object"],
        "summary": "Get Example Data (Object)",
        "operationId": "getExampleDataDataObject",
        "parameters": [],
        "responses": {
          "200": {
            "description": "Successful Response",
            "content": {
              "application/json": {
                "schema": {
                  "title": "anyOf bug reproduction",
                  "type": "object",
                  "additionalProperties": {
                    "anyOf": [{ "type": "string" }, { "type": "number" }]
                  }
                }
              }
            }
          }
        }
      }
    },
    "/example-array": {
      "get": {
        "tags": ["array"],
        "summary": "Get Example Data (Array)",
        "operationId": "getExampleDataDataArray",
        "parameters": [],
        "responses": {
          "200": {
            "description": "Successful Response",
            "content": {
              "application/json": {
                "schema": {
                  "title": "anyOf bug reproduction",
                  "type": "array",
                  "items": {
                    "anyOf": [{ "type": "string" }, { "type": "number" }]
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

Full reproduction here: https://github.com/phlickey/openapigenerator-ts-anyof_bug_reproduction

Generation Details

npx openapi-generator-cli generate -g typescript -o output -i spec.json

Steps to reproduce

Reproduced here: https://github.com/phlickey/openapigenerator-ts-anyof_bug_reproduction

Related issues/PRs

Suggest a fix

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/typescript/api/api.mustache#L20-L22

When defining an api value as a union of two or more primitive types (ie string | number | null) the import line could be skipped. This presumably gets more difficult if {{ className }} is being passed as a union of generated and primitive types.

Start with modules/openapi-generator/src/main/resources/typescript/api/api.mustache at lines 20-22 and the provided minimal OpenAPI specification. Run the npx openapi-generator-cli command from the issue using spec.json, then inspect generated TypeScript imports for anyOf values such as string | number. Done means the generated imports are valid and primitive-only unions do not produce malformed model imports.