[#27420] docdb: Fix flaky test MasterPathHandlersItest.TestClusterBalancerWarnings

Summary:
This one is the same issue as D44361. After D43208, the advisory lock table and transactions table are now created by CatalogManagerBgTasks. In slower builds (e.g., TSAN or ASAN), these tables might be created while only 2 out of 3 tservers are live. This can lead to them initially being created with only 2 replicas, making them under-replicated. The Load Balancer will then later handle the under replicated tablet.
Thus, we need more wait time before checking the Warnings Summary table to account for the time required by the Load Balancer to process the under-replicated advisory lock and transaction tablets.
The test was previously flaky due to insufficient wait time, as seen in the failure below:

```
/share/jenkins/workspace/github-yugabyte-db-alma8-master-clang19-tsan/yugabyte-db/src/yb/integration-tests/master_path_handlers-itest.cc:1751
Expected equality of these values:
rows.size()
Which is: 0
1
```
Jira: DB-16959

Test Plan: ./yb_build.sh asan --cxx-test integration-tests_master_path_handlers-itest --gtest_filter MasterPathHandlersItest.TestClusterBalancerWarnings

Reviewers: asrivastava

Reviewed By: asrivastava

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D44377

Author: yusong-yan

src/yb/integration-tests/master_path_handlers-itest.cc

@@ -1746,13 +1746,21 @@ TEST_F(MasterPathHandlersItest, TestClusterBalancerWarnings) {
   auto hp = HostPort::FromBoundEndpoint(cluster_->mini_tablet_server(0)->bound_rpc_addr());
   ASSERT_OK(yb_admin_client_->ChangeBlacklist({hp}, true /* add */, false /* blacklist_leader */));
 
-  SleepFor(FLAGS_catalog_manager_bg_task_wait_ms * 2ms); // Let the load balancer run once
-  auto rows = ASSERT_RESULT(GetHtmlTableRows("/load-distribution", "Warnings Summary"));
-  ASSERT_EQ(rows.size(), 1);
-  ASSERT_EQ(rows[0].size(), 2);
-  ASSERT_STR_CONTAINS(rows[0][0], "Could not find a valid tserver to host tablet");
-  // 3 user tablets + system tablets
-  auto tablet_count = std::stoi(rows[0][1]);
+  ANNOTATE_UNPROTECTED_WRITE(FLAGS_TEST_sleep_before_reporting_lb_ui_ms) = 500;
+  std::vector<std::string> row;
+  ASSERT_OK(WaitFor([&]() -> Result<bool> {
+    auto rows = VERIFY_RESULT(GetHtmlTableRows("/load-distribution", "Warnings Summary"));
+    if (rows.empty()) {
+      return false;
+    }
+    SCHECK_EQ(rows.size(), 1, IllegalState, "Expected one row");
+    row = rows[0];
+    return true;
+  }, 10s /* timeout */, "Waiting for warnings to show up in the Warnings Summary table"));
+
+  ASSERT_EQ(row.size(), 2);
+  ASSERT_STR_CONTAINS(row[0], "Could not find a valid tserver to host tablet");
+  auto tablet_count = std::stoi(row[1]);
   ASSERT_GT(tablet_count, 3);
 }