test: unit tests for the services extracted in #597

[Zeroupper]

Update (post-#597 merge)

test/services_test.dart now covers most of the originally-scoped services:

• MessageService — refresh sort/count, failure keeps old list, polling idempotency + stop, dispose safety
• AuthService — signed-out vs signed-in username, auth-state delegation, invitation filtering
• ConsentService — backend-vs-local branching, caching, accept-notifies, backend failure paths
• StudyService — capability queries, deploy/start no-ops, persistence, retryable configure

The originally-cited gaps (hasInformedConsentBeenAccepted backend-vs-local branching, message-refresh failure) are now asserted.

Remaining work

• Add an explicit ≥80% line-coverage assertion/gate per extracted service (currently behaviourally covered, not measured)
• Extract shared fixtures/mocks into a common test harness (dedupe across services_test.dart / view_models_test.dart)

Effort: S · Priority: P1 · Change risk: None