[EPIC] Test foundation #643

The #597 service-extraction EPIC (#589–#599) is now merged, and the test suite has grown well beyond the original "one unit test": test/ now holds ~51 passing tests across services_test.dart, view_models_test.dart, app_bloc_test.dart, cams_app_test.dart, plus heart_rate_data_model_test.dart.

This EPIC now tracks the remaining gaps on top of that foundation so the other EPICs can land safely:

• Formalize per-service coverage assertions and shared fixtures (test: unit tests for the services extracted in #597 #644)
• Stream-driven tests for the card view-models, once the leak EPIC ([EPIC] Fix leaked timers, stream subscriptions, and lifecycle listeners #613) fixes single-subscription behavior (test: stream-driven tests for card view-models #645)
• Widget tests for the router redirect + consent gate — currently untested (test: widget tests for the router redirect and consent gate #646)
• A CI coverage floor so the suite can only grow (ci: run tests with a coverage floor on every PR (refs #585) #647)

Definition of done: every refactored view-model and the two critical flows (login redirect, consent gate) have tests running in CI with a ratchet-only coverage floor.

Note: original body referenced branch chore/cams-2.x-upgrade @ 560a181; the service refactor has since merged to the mainline.