[#26979] docdb: Disable load balancing for PgSharedMemTest.LongRead

Summary:
Issue:
`PgCronTest.CancelJobOnLeaderChange` might fail when new system tablets are added on tserver - This can occur, for example, when:
* Enabling `ysql_yb_enable_advisory_locks`,  creates an advisory lock system table
* Enabling `ysql_enable_auto_analyze_infra`,  creates a stateful service table.
The test performs a long-running read by sleeping for `FLAGS_TEST_transactional_read_delay_ms` (65 seconds) before executing the actual read. During this period, the tablet leader may move to a different node because the newly added system tablets trigger load balancing. After the sleep completes, the system detects the leader change and retries the read on the new leader. However, since each long read takes over 65 seconds and the read timeout is 120 seconds, two consecutive long reads can exceed the timeout limit, resulting in test failure.

Fix:
Disable load balancing during the test to prevent unexpected leader moves during long reads.
Jira: DB-16439

Test Plan: ./yb_build.sh --cxx-test pgwrapper_pg_shared_mem-test --gtest_filter PgSharedMemTest.LongRead

Reviewers: sergei, rthallam

Reviewed By: sergei, rthallam

Subscribers: rthallam, ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D43567

Author: yusong-yan

src/yb/yql/pgwrapper/pg_shared_mem-test.cc

@@ -28,6 +28,7 @@
 
 using namespace std::literals;
 
+DECLARE_bool(enable_load_balancing);
 DECLARE_bool(pg_client_use_shared_memory);
 DECLARE_bool(TEST_pg_client_crash_on_shared_memory_send);
 DECLARE_bool(TEST_skip_remove_tserver_shared_memory_object);
@@ -198,6 +199,13 @@ class PgSharedMemBigTimeoutTest : public PgSharedMemTest {
 };
 
 TEST_F_EX(PgSharedMemTest, LongRead, PgSharedMemBigTimeoutTest) {
+  // Disable load balancing, as tablet leader move might happen during the long-running read
+  // by load balancer and causing test to fail, here is the steps:
+  // 1. Perform long read and start sleep FLAGS_TEST_transactional_read_delay_ms (65 seconds)
+  // 2. During this time, the tablet leader is moved by the load balancer
+  // 3. After the 65s sleep, it detects the leader change retries the read on the new leader
+  // 4. The retried read also sleeps for 65 seconds. Combined, the total read time exceeds the 120s
+  ANNOTATE_UNPROTECTED_WRITE(FLAGS_enable_load_balancing) = false;
   auto conn = ASSERT_RESULT(Connect());
 
   ASSERT_OK(conn.Execute("CREATE TABLE t (key INT PRIMARY KEY) SPLIT INTO 1 TABLETS"));