[Bug]: TypeError: Promises are not supported under tools.rspack which contradicts rsbuild documentation

[jjoshi-space]

Version

System:
    OS: Windows 11 10.0.22631
    CPU: (14) x64 Intel(R) Core(TM) Ultra 7 155U
    Memory: 41.53 GB / 63.52 GB
  Browsers:
    Edge: Chromium (131.0.2903.63)
    Internet Explorer: 11.0.22621.3527
  npmPackages:
    storybook-vue3-rsbuild: ^1.0.1 => 1.0.1

Details

Hello team,
I'm trying to run some async code to build our custom rspackConfig and eventually use mergeConfig
I've followed this documentation, and have come across below issue

TypeError: Promises are not supported
    at .\node_modules\@rsbuild\core\dist\index.cjs:502:52

I cannot use rsbuildFinal to run async code as it throws error upon using Webpack based plugins within rspackConfig, and guides to pass the same into tools: { rspack: () => {} }

Can the team confirm whether we are allowed to use async under tools.rspack as per below documentation
tools.rspack can also be an async function:

Reproduce link

https://codesandbox.io/p/devbox/vs82ny

Reproduce Steps

1. Checkout the rsbuild.config.mjs file --> tools: rspack async method
2. Execute npm run storybook
3. Check the logs displaying TypeError message

[fi3ework]

Seems there's already a left TODO of the same issue in the codebase https://github.com/mdrobny/storybook-rsbuild/blob/6e947e5967d3284b0fa229f7133603837feb4995/packages/builder-rsbuild/src/index.ts#L75.