Add edge case tests for refreshOverdueNextPayments guards

[Copilot]

The existing test for refreshOverdueNextPayments only verified the happy path. The implementation has guards at lines 221-223 that skip execution when subscriptions are loading or empty, but these weren't tested.

Changes

• Added test for empty subscriptions list guard
• Added test for subscriptions loading state guard

Both tests verify the use case is never invoked when guard conditions are met:

test(
  'refreshOverdueNextPayments does nothing when subscriptions are empty',
  () async {
    subscriptionsController.add(const []);
    // ... setup streams
    await viewModel.refreshOverdueNextPayments();
    
    verifyNever(() => refreshOverdueNextPaymentsUseCase(any()));
  },
);

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.