PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

When the CLI compiles comments and types into Swagger decorators, it omits the async keyword in arrow functions that reference another DTO, even though those functions use the await keyword.
This change ensures that any arrow function using await within a DTO reference property is correctly prefixed with the async keyword.

Does this PR introduce a breaking change?

• Yes
• No

Other information

{
  // ...
  "compilerOptions": {
    "plugins": [{ "name": "@nestjs/swagger", "options": { "introspectComments": true, "esmCompatible": true } }]
  },
  // ...
}

import { Exclude, Expose } from 'class-transformer';

@Exclude()
export class Foo {
  /**
   * Foo
   */
  @Expose()
  bar!: Bar;
}

@Exclude()
export class Bar {
  /**
   * Property
   */
  @Expose()
  property!: string;
}

let Foo = class Foo {
    static _OPENAPI_METADATA_FACTORY() {
        return { bar: { required: true, type: () => (await import("./foo-bar.dto.js")).Bar, description: "Foo" } }; // There is no 'async' keyword at the arrow function.
    }
};