test(cache_first): update test for new yield-after-fetch semantics

The cacheFirst fix in 5511f87 added a yield after fetchAndPopulate
so streams never hang when populate is a no-op for this filter
(the liked-tab spinner-forever bug). Test expectation updated: the
first emission after an empty drift is now the still-empty yield
("we tried, nothing to show yet"), and the simulated drift re-emit
yields the populated rows as the second emission.

flutter_client/test/cache/cache_first_test.dart

@@ -32,9 +32,12 @@ void main() {
       isOnline: () async => true,
     );
 
-    final firstFuture = results.first;
-    controller.add([]);
-    expect(await firstFuture, 42);
+    // Post-coldFetchAttempted guard, cacheFirst yields the current
+    // (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();
+    expect(emissions, [0, 42]);
     expect(fetchCalled, 1);
     await controller.close();
   });