feat(example): cover full API surface in configurable demo

- Extract `ExampleConfig` to replace the individual demo configuration
  parameters.
- Add controls for reverse, separators, scrollbars, pull-to-refresh,
  initial page index, load-more threshold, `useRealItemIndex`, error
  simulation, and retry behavior.
- Drive resets through `ScrollInfinityController` and use `dev.log` for
  `onError` and `onItemsLoaded` callbacks.
- Make mock data deterministic with alternating failure modes and a
  fixed five-page limit.
- Replace the placeholder widget test with coverage for configuration,
  layout, controller behavior, and pagination scenarios.

Author: dariomatias-dev