Describe the bug

After upgrading from v4.0.18 to v4.1.0 (specifically I found that this issue was introduced in v4.1.0-beta.5) my integration tests no longer work because the "pg" module throws an error:

TypeError: Class extends value [object Module] is not a constructor or null
 ❯ poolFactory node_modules/pg/lib/index.js:14:3
 ❯ new PG node_modules/pg/lib/index.js:25:15
 ❯ Object.<anonymous> node_modules/pg/lib/index.js:50:18

If you investigate the source files, you will see that the Pool exported from "pg" is a factory function that returns a class that extends the Pool class from "pg-pool". This leads me to believe that the underlying issue has something to do with module resolution. It could be something like cjs getting wrapped to fit esm or something. However, I tried importing different entrypoints from "pg" but to no avail.

You will notice that the reproduction runs with the nuxt/vue test utils. The vitest config uses defineVitestProject from @nuxt/test-utils, so the issue could also be there but since the issue first started appearing after updating vitest, it would also be reasonable to assume that this a regression but it could very well be some sort of incompatibility.

Thank you for your time and work. It's much appreciated :)

Reproduction

1. Clone this repository: https://github.com/konstantin-lukas/vitest-4.1.0-repro
2. run npm i to install deps
3. run npm test to run vitest
4. You should see the described error

System Info

System:
    OS: Linux 6.6 Ubuntu 24.04.4 LTS 24.04.4 LTS (Noble Numbat)
    CPU: (16) x64 AMD Ryzen 7 7700X 8-Core Processor
    Memory: 10.20 GB / 15.20 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 24.14.1 - /home/konst/.nvm/versions/node/v24.14.1/bin/node
    npm: 11.11.0 - /home/konst/.nvm/versions/node/v24.14.1/bin/npm
  npmPackages:
    vitest: ^4.1.2 => 4.1.2

Used Package Manager

npm

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