KoolbaseTestData class

Records for KoolbaseCollectionList and KoolbaseRecordView in widget tests, so a screen can be pumped POPULATED rather than stuck loading.

Certification and testing infrastructure -- NOT a local-data API. It replaces only the data SOURCE: the list's base query and the record view's fetch. The controllers, paging, states and rendering after that are the production code, so what renders here is what renders in an app.

KoolbaseTestData(
  collections: {
    'songs': [
      {'id': 'cert-1', 'title': 'A song', 'coverUrl': 'https://example.com/c.png'},
    ],
  },
  child: const SongsScreen(),
)

A list pages like the server: its limit and offset are honoured and the total is exact, so Load more appears when it would. Filters and order are NOT applied: records come in the order given. A record view finds its record by id, and an unknown id throws KoolbaseNotFoundException, as the API answers.