Unit tests for ScenarioDeepLinkHandler URI building

go_router's GoRouter cannot be easily instantiated in unit tests — use a mock class that implements or wraps the router interface, or extract navigation logic behind a NavigationService interface that can be mocked. If go_router version does not support easy mocking, consider defining a thin RouterDelegate abstraction in ScenarioDeepLinkHandler that the tests can replace. URI building should use Dart's Uri class (Uri(path: ..., queryParameters: {...})) rather than string interpolation to guarantee correct encoding. For cold-start vs warm-start distinction, the handler likely checks whether GoRouter.of(context) is available or uses a platform channel callback — mock accordingly.

Unit tests only using flutter_test and mocktail. Create scenario_deep_link_handler_test.dart. Mock both the go_router GoRouter instance and PromptHistoryRepository. For URI correctness tests: parse the resulting URI string using Uri.parse() and assert scheme, path, and queryParameters individually — do not do string equality on the full URI (fragile).

For URL-encoding tests: include at least one Norwegian character (ø, æ, å) and one ampersand in prefill values and assert the encoded form. For cold/warm-start: mock the router state (null vs active) and assert the correct navigation method (push vs go) is called. For validation failures: stub the repository to return null or throw an access-denied exception and assert fallback URI is used.