DataSource contracts

Prefer one contract per technical responsibility:

abstract interface class CatalogRemoteDataSource implements DataSource {
  Future<CatalogResponseDto> fetchCatalog();
}

abstract interface class CatalogCacheDataSource implements DataSource {
  Future<CatalogCacheDto?> readCatalog();
  Future<void> writeCatalog(CatalogCacheDto data);
}

DataSourceContainer copies the supplied iterable and resolves by Dart type assignability.

Strict ambiguity handling prevents registration order from silently selecting storage behavior. If remote and local sources have different responsibilities, give them different interfaces instead of a runtime string key.