fix(test): capture cacheFirst stream Future before feeding controller

flutter_client/test/cache/cache_first_test.dart

@@ -36,7 +36,11 @@ void main() {
     // (still-empty) rows after the fetch returns, so the stream
     // never hangs when populate is a no-op for this filter. The
     // simulated drift re-emit then yields the populated rows.
-    final emissions = await results.take(2).toList();
+    // Capture the Future *before* feeding the controller so the
+    // listener is subscribed when the first add() lands.
+    final emissionsFuture = results.take(2).toList();
+    controller.add([]);
+    final emissions = await emissionsFuture;
     expect(emissions, [0, 42]);
     expect(fetchCalled, 1);
     await controller.close();