[BUG][DART] oneOf dart code generation: wrong types and html-encoded code

Bug Report Checklist

• [ Y] Have you provided a full/minimal spec to reproduce the issue?
• [ Y] Have you validated the input using an OpenAPI validator (example)?
• [ Y] What's the version of OpenAPI Generator used?
• [Y ] Have you search for related issues/PRs?
• [ Y] What's the actual output vs expected output?
• [ -] [Optional] Bounty to sponsor the fix ([example]

Description

When using oneOf, with options for String or int, the generated dart code produces the class model with a few errors:

1: OneOf<string, integer> : where "string" should be "String", and "integer" should be "int". These are produced as expected (String or int) with other simple entries.

part of openapi.api;
class Pet {
  OneOf<string,integer> bodyText = null;

2: The generated code has encoded the "<>" symbols in html form (<, >)

    if (json['bodyText'] == null) {
      bodyText = null;
    } else {
      bodyText = OneOf<string,integer>.fromJson(json['bodyText']);

openapi-generator version

$ openapi-generator version
4.1.2

OpenAPI declaration file content or url

openapi: 3.0.1

    Blob:
      required:
      - name
      type: object
      properties:
        bodyText:
          oneOf:
            - type: string
            - type: integer

Command line used for generation

openapi-generator generate -i oneof.yaml -g dart -o oneof-out3a -p browserClient=false,supportDart2=true,generateAliasAsModel=true

Steps to reproduce

1: Use the default pet store model at https://editor.swagger.io/
2: Add these 4 lines to a model's properties:

        bodyText:
          oneOf:
            - type: string
            - type: integer

3: Generate code in the manner shown above
4: Observe lib/model/.dart code

Related issues/PRs

#3884

Suggest a fix

If I knew, I'd drop a PR :)

Start by reproducing the issue with the provided oneof.yaml and the openapi-generator generate command, then inspect the generated lib/model/.dart file. Compare the oneOf<String, int> declaration and deserialization output with the reported HTML-encoded version. Done means the generated Dart model uses String and int and emits unencoded angle brackets.