Unify previous source resolution

internal/adapters/storage/fake.go

@@ -42,6 +42,7 @@ type FakeUploadCall struct {
 type FakeDownloadCall struct {
 	Key       string
 	LocalPath string
+	Bytes     int64
 }
 
 // FakeObject is a deterministic fake object-store record.
@@ -151,12 +152,13 @@ func (f *FakeBackend) DownloadTo(ctx context.Context, key string, destination io
 		return err
 	}
 	defer source.Close()
-	f.mu.Lock()
-	f.Downloads = append(f.Downloads, FakeDownloadCall{Key: normalizeObjectKey(key)})
-	f.mu.Unlock()
-	if _, err := io.Copy(destination, source); err != nil {
+	count, err := io.Copy(destination, source)
+	if err != nil {
 		return fmt.Errorf("download object %q: write destination: %w", key, err)
 	}
+	f.mu.Lock()
+	f.Downloads = append(f.Downloads, FakeDownloadCall{Key: normalizeObjectKey(key), Bytes: count})
+	f.mu.Unlock()
 	return nil
 }