`vi.mock` doesn't seem to work inside a sandbox directory

Describe the bug

StrykerJS creates a sandbox directory for your project by default. Inside that sandbox directory, it copies all project files while mutating your production code. However when running vi.mock it doesn't seem to work because of this sandboxing mechanism. I've reproduced it using just vitest (without StrykerJS).

Note: I'm sure the "sandboxing" is the issue here. StrykerJS also supports --inPlace mode, that disables sandboxing and I can confirm that Stryker works as expected in that case.

Reproduction

issue.zip

1. Unzip the issue.zip inside a directory
2. cd into the root of the directory
3. run npm i
4. run npx vitest. See that it works.
5. run cd .stryker-tmp/sandbox-aGW0K0 (this is a copy of the project, but instrumented with mutations)
6. again, use npx vitest. You can now see the error: TypeError: [Function Discount] is not a spy or a call to a spy!

The console outputs includes:

Error: [MSW] Failed to register a Service Worker for scope ('http://localhost:63315/') with script ('http://localhost:63315/mockServiceWorker.js'): Service Worker script does not exist at the given path.

Did you forget to run "npx msw init <PUBLIC_DIR>"?

Learn more about creating the Service Worker script: https://mswjs.io/docs/cli/init

System Info

System:
    OS: Linux 5.15 Ubuntu 20.04.6 LTS (Focal Fossa)
    CPU: (16) x64 11th Gen Intel(R) Core(TM) i7-11850H @ 2.50GHz
    Memory: 5.79 GB / 15.49 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 22.9.0 - ~/.nvm/versions/node/v22.9.0/bin/node
    npm: 11.0.0 - ~/.nvm/versions/node/v22.9.0/bin/npm
    pnpm: 9.15.1 - ~/.nvm/versions/node/v22.9.0/bin/pnpm
  Browsers:
    Chrome: 131.0.6778.139

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the attached issue.zip reproduction: install dependencies, run npx vitest from the project root, then repeat from the .stryker-tmp/sandbox-aGW0K0 copy. Compare the two runs and investigate why vi.mock fails in the sandbox; done means the copied project runs without the spy error and the reported MSW failure is addressed or understood.