capture-api-response-test-fixture

### API Response Test Fixtures

For provider response parsing tests, we aim at storing test fixtures with the true responses from the providers (unless they are too large in which case some cutting that does not change semantics is advised).

The fixtures are stored in a `__fixtures__` subfolder, e.g. `packages/openai/src/responses/__fixtures__`. See the file names in `packages/openai/src/responses/__fixtures__` for naming conventions and `packages/openai/src/responses/openai-responses-language-model.test.ts` for how to set up test helpers.