[BUG][typescript-axios] Allof keyword makes nullable fields become non-nullable

Description

When using openapi-generator 7.12.0 with OpenAPI 3.1.0 specification, fields marked as nullable in the referred schema are not properly generated as nullable in the TypeScript output, if the schema is combined using the allOf keyword. The same specification works correctly when using OpenAPI 3.0.0.

When a field is defined with type: [string, "null"] in schema in OpenAPI 3.1.0, if the schema is combined using allOf keyword, the generated TypeScript interface does not maintain the nullable property in the generated code.

openapi-generator version

7.12.0

OpenAPI declaration file content

openapi: 3.1.0
info:
  version: 1.0.0
  title: Example - OpenAPI 3.1
  description: Example

components:
  schemas:
    TargetSchema:
      properties:
        id:
          type: [string, "null"]
        name:
          type: string
    RefSchemaWithAllOf:
      allOf:
        - $ref: "#/components/schemas/TargetSchema"

Generated API

/**
 * 
 * @export
 * @interface RefSchemaWithAllOf
 */
export interface RefSchemaWithAllOf {
    /**
     * 
     * @type {string}
     * @memberof RefSchemaWithAllOf
     */
    'id'?: string;
    /**
     * 
     * @type {string}
     * @memberof RefSchemaWithAllOf
     */
    'name'?: string;
}
/**
 * 
 * @export
 * @interface TargetSchema
 */
export interface TargetSchema {
    /**
     * 
     * @type {string}
     * @memberof TargetSchema
     */
    'id'?: string | null;
    /**
     * 
     * @type {string}
     * @memberof TargetSchema
     */
    'name'?: string;
}

Generation Details

Using CLI:

openapi-generator-cli generate -g typescript-axios -i swagger.yml

Steps to reproduce

1. Create the OpenAPI 3.1.0 specification as shown above
2. Generate TypeScript code using openapi-generator 7.12.0
3. Observe the generated output

Current output:

export interface RefSchemaWithAllOf {
    'id'?: string;  // Should be string | null
    'name'?: string;
}

Expected output:

export interface RefSchemaWithAllOf {
    'id'?: string | null;
    'name'?: string;
}

Start by reproducing the issue with the provided OpenAPI 3.1.0 declaration and the typescript-axios generator command. Then trace the generator's allOf handling for referenced schemas and compare it with the direct TargetSchema output. Done means the generated RefSchemaWithAllOf interface preserves id as string | null, with regression coverage for the example.