🐛 Knip crashes when `testEnvironment` uses the object form in `rstest` config

[yslpn]

Prerequisites

• I'm using the latest version
• I've read the relevant documentation
• I've searched for existing issues
• I've checked the list of known issues
• I've read the issue reproduction guide

Reproduction url

Not available yet.

Reproduction access

• I've made sure the reproduction is publicly accessible

Description of the issue

I wanted to enable Rstest prebundling for the jsdom test environment by adding
the new prebundle option:

import { defineConfig } from '@rstest/core';

export default defineConfig({
  testEnvironment: {
    name: 'jsdom',
    prebundle: 'auto',
  },
});

After changing testEnvironment from the string form to the object form,
running Knip crashes before it can report its analysis results.

Steps to reproduce

1. Use @rstest/core 0.11.9 and knip 6.24.0.

2. Add the object-form testEnvironment configuration shown above to
   rstest.config.ts.

3. Run:

   pnpm run knip

Actual result

Knip terminates with an uncaught TypeError:

TypeError: Cannot read properties of undefined (reading 'startsWith')
    at .../knip/dist/WorkspaceWorker.js:410:54
    at Array.some (<anonymous>)
    at runPlugin (.../knip/dist/WorkspaceWorker.js:410:54)
    at async WorkspaceWorker.runPlugins (.../knip/dist/WorkspaceWorker.js:421:33)
    at async build (.../knip/dist/graph/build.js:147:35)
    at async run (.../knip/dist/run.js:40:109)
    at async main (.../knip/dist/cli.js:34:29)

[ELIFECYCLE] Command failed with exit code 1.

Expected result

Knip should complete successfully when Rstest's supported object form of
testEnvironment is used, or report a clear compatibility/configuration error
instead of crashing.

Environment

• Node.js 24.16.0
• pnpm 11.7.0
• @rstest/core 0.11.9
• knip 6.24.0