@next/mdx: unhandled promise rejection when plugin path fails to resolve in mdx-js-loader

Inspect packages/next-mdx/mdx-js-loader.js around line 61, starting with the getOptions() promise and its interaction with the webpack loader callback. Run the linked reproduction with npm install and npm run build. Done means an invalid plugin path produces a clear webpack-reported module-resolution error instead of an unhandled rejection or hanging build.

Link to the code that reproduces this issue

https://github.com/Jinoko01/mdx-catch-bug

To Reproduce

1. Clone the reproduction repo
2. Run npm install
3. Run npm run build (next build --webpack)
4. Observe the build hangs indefinitely or throws UnhandledPromiseRejection

Current vs. Expected behavior

Current:
The build process hangs without any error message, or Node.js throws
an UnhandledPromiseRejection without webpack properly reporting the error.

node:internal/process/promises:288
UnhandledPromiseRejection: Cannot find module 'remark-plugin-does-not-exist'

Expected:
Webpack should receive the error via callback and report it clearly:

Error: Cannot find module 'remark-plugin-does-not-exist'
Require stack: ...

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Enterprise
  Available memory (MB): 32488
  Available CPU cores: 16
Binaries:
  Node: 24.14.0
  npm: 11.9.0
  Yarn: N/A
  pnpm: 11.1.1
Relevant Packages:
  next: 16.3.0-canary.29 // Latest available version is detected (16.3.0-canary.29).
  eslint-config-next: N/A
  react: 19.2.6
  react-dom: 19.2.6
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Markdown (MDX), Webpack

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

Root cause: In packages/next-mdx/mdx-js-loader.js, the Promise returned by getOptions() is missing a .catch() handler. When a plugin string path
fails to resolve (e.g., module not found, ESM import failure), the rejection
is never forwarded to webpack's callback, causing the build to stall.

// Current code (line 61)
getOptions(options, this.context).then((userProvidedMdxOptions) => {
mdxLoader.call(proxy, ...args)
})
// Missing: .catch((error) => callback(error))