I needed to add a jest-setup.ts with the import '@testing-library/jest-dom' to make the test runner happy.

The missing piece for my IDE (Rider) was to add that jest-setup.ts to the include array in my tsconfig.json:

  "include": ["src/**/*", "jest-setup.ts"]

as documented here: https://github.com/testing-library/jest-dom?tab=readme-ov-file#with-typescript