test: add tests for fixtures, mock fetch, mock db, and happy-dom
- Add comprehensive fixture shape tests.
- Expand mockFetch to support methods, status codes, and body tracking.
- Introduce getFetchCalls to inspect intercepted requests.
- Add mockDb helper for in-memory DB operations.
- Reorganize tests for clarity and coverage.
- Ensure happy-dom setup/cleanup works correctly.
Add support for HTTP methods, status codes, body and headers in the fetch
mock. Track calls and expose getFetchCalls for assertions. Update fixture
interfaces to use string IDs, add ImportResult and ApiError types, and
provide standard error constants. Adjust fixture data to match new types.
djalim