[BUG] typescript-angular discriminator mapping ignored in interfaces

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?

Description

Hi,
It seems that the typescript-angular generator is ignoring the mapping as discriminator value on the children interfaces.

openapi-generator version

  <version>5.1.0-SNAPSHOT</version>
  Commit: 8cb4741248ecf0baaf3c608952290fb21c231490
  Commit Date: lundi 8 mars 2021 18:11:50
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

If we take the following json file (named pets.json)

{
	"openapi": "3.0.1",
	"info": {
		"title": "Accruals",
		"version": "accruals"
	},
	"paths": {},
	"components": {
		"schemas": {
			"Pet": {
				"type": "object",
				"required": [
					"petType"
				],
				"properties": {
					"id": {
						"title": "Int32",
						"type": "integer",
						"format": "int32"
					},
					"petType": {
						"type": "string"
					}
				},
				"discriminator": {
					"propertyName": "petType",
					"mapping": {
						"MapCatType": "#/components/schemas/Cat",
						"MapDogType": "#/components/schemas/Dog",
						"MapLizardType": "#/components/schemas/Lizard"
					}
				}
			},
			"Cat": {
				"allOf": [
					{
						"$ref": "#/components/schemas/Pet"
					},
					{
						"type": "object",
						"properties": {
							"name": {
								"type": "string"
							}
						}
					}
				]
			},
			"Dog": {
				"allOf": [
					{
						"$ref": "#/components/schemas/Pet"
					},
					{
						"type": "object",
						"properties": {
							"bark": {
								"type": "string"
							}
						}
					}
				]
			},
			"Lizard": {
				"x-discriminator-value": "LizardTypeX",
				"allOf": [
					{
						"$ref": "#/components/schemas/Pet"
					},
					{
						"type": "object",
						"properties": {
							"lovesRocks": {
								"type": "boolean"
							}
						}
					}
				]
			}
		}
	}
}

Generation Details

It happens with the typescript-angular generator with the additionnal-property taggedUnions=true.

In the output folder we have in the model/pet.ts file (as espected).

import { Lizard } from './lizard';
import { Cat } from './cat';
import { Dog } from './dog';


export type Pet = Lizard | Cat | Dog;

But int the model/cat.ts, we have the petType as Cat instead of MapCatType supposed.

export interface Cat { 
    id?: number;
    petType: 'Cat';
    name?: string;
}

instead of

export interface Cat { 
    id?: number;
    petType: 'MapCatType';
    name?: string;
}

Notice that in the Lizard interface, petType is setted as LizardTypeX thanks to "x-discriminator-value": "LizardTypeX" (as espected too).

export interface Lizard { 
    id?: number;
    petType: 'LizardTypeX';
    lovesRocks?: boolean;
}

Steps to reproduce

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i ./pets.json -g typescript-angular -o ./generated --additional-properties=taggedUnions=true

Suggest a fix

I've forked the repository and created a branch with a fix where I've made (on the TypeScriptAngularClientCodegen.java file).
I override the discriminatorValue property if there is a mapping to each children interface (expect if an x-discriminator-value is already setted).
https://github.com/vergerf/openapi-generator/tree/typescript-angular-discriminator-mapping

EDIT: link to the PR :
https://github.com/OpenAPITools/openapi-generator/pull/8929

Start with the provided pets.json and run the typescript-angular generation command with taggedUnions=true. Inspect TypeScriptAngularClientCodegen.java and compare the generated Cat interface with the discriminator mapping, while preserving the existing Lizard x-discriminator-value behavior. The linked pull request contains the proposed fix and should be reviewed before starting new work.