[#21772] YSQL: Stabilize createIndexViolatingUniqueness with Connection Manager enabled

Summary:
When connection manager is enabled, the test org.yb.pgsql.TestPgUniqueConstraint.createIndexViolatingUniqueness fails.

During the runtime of the test, the allowed preview flags are modified to a certain value. This disables the required preview flag to enable connection manager, hence causing the test to fail.

The corresponding flag (enable_ysql_conn_mgr) can be added to the preview flag list when the connection endpoint is the connection manager layer.

Jira: DB-10648

Test Plan:
Jenkins: enable connection manager, test regex: .*TestPgUniqueConstraint.*

./yb_build.sh --enable-ysql-conn-mgr-test --java-test org.yb.pgsql.TestPgUniqueConstraint#createIndexViolatingUniqueness

Reviewers: janand, myang

Reviewed By: janand, myang

Differential Revision: https://phorge.dev.yugabyte.com/D33467

Author: rahulb-yb

java/yb-pgsql/src/test/java/org/yb/pgsql/TestPgUniqueConstraint.java

@@ -402,8 +402,12 @@ public void createIndexViolatingUniqueness() throws Exception {
           "enable_transactional_ddl_gc", "false"));
     }
     for (HostAndPort hp : miniCluster.getTabletServers().keySet()) {
-      assertTrue(miniCluster.getClient().setFlag(hp,
-          "allowed_preview_flags_csv", "ysql_yb_ddl_rollback_enabled=true"));
+      if (isTestRunningWithConnectionManager())
+        assertTrue(miniCluster.getClient().setFlag(hp,
+            "allowed_preview_flags_csv", "ysql_yb_ddl_rollback_enabled,enable_ysql_conn_mgr"));
+      else
+        assertTrue(miniCluster.getClient().setFlag(hp,
+            "allowed_preview_flags_csv", "ysql_yb_ddl_rollback_enabled=true"));
       assertTrue(miniCluster.getClient().setFlag(hp,
           "ysql_yb_ddl_rollback_enabled", "false"));
     }