test(uploads): a slow heartbeat write, so one is in flight when the job ends

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/uploads/worker_test.go

@@ -25,6 +25,9 @@ type wStore struct {
 	startErr  error
 	saves     [][]domain.ProgressStep
 	beats     int
+	// beatDelay makes each heartbeat write slow, so one is still in flight when a job
+	// ends -- which is what proves the worker waits for its heartbeat to finish.
+	beatDelay time.Duration
 }
 
 func newWStore() *wStore {
@@ -60,6 +63,7 @@ func (s *wStore) SaveProgress(_ context.Context, _, _ string, steps []domain.Pro
 }
 
 func (s *wStore) Heartbeat(context.Context, string, string) error {
+	time.Sleep(s.beatDelay)
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	s.beats++