[#28881] docdb: Fix PgRowLockTest.SelectForKeyShareWithRestart for enable txn ddl

Summary:
Context:
This test verifies that SELECT FOR KEY SHARE doesn't cause crashes during cluster restart(From D27662)
Issue:
With `ysql_yb_ddl_transaction_block_enabled` now enabled by default, the first test run `DROP TABLE` statement at the end is side the explicit user-defined transaction block (started for `SELECT FOR KEY SHARE`), which is not committed before the cluster restart. This prevents proper table cleanup, causing the second test run to fail with error:
```
2025-10-09 02:50:35.764 UTC [547047] ERROR:  relation "foo" already exists
2025-10-09 02:50:35.764 UTC [547047] STATEMENT:  CREATE TABLE foo(k INT, v INT)
../../src/yb/yql/pgwrapper/pg_row_lock-test.cc:224: Failure
Failed
Bad status: Network error (yb/yql/pgwrapper/libpq_utils.cc:457): Execute of 'CREATE TABLE foo(k INT, v INT)' failed: 7, message: ERROR:  relation "foo" already exists (pgsql error 42P07) (aux msg ERROR:  relation "foo" already exists)
```
Fix:
Use `DROP TABLE IF EXISTS` at the beginning, which maintains the test's original purpose of verifying SELECT FOR KEY SHARE doesn't cause crashes while also resolving the DDL transaction blocking issue by ensuring each run starts with a clean state, this avoids the `ERROR:  relation "foo" already exists` test crash
Jira: DB-18603

Test Plan: ./yb_build.sh --cxx-test pgwrapper_pg_row_lock-test --gtest_filter PgRowLockTest.SelectForKeyShareWithRestart

Reviewers: bkolagani, rthallam

Reviewed By: bkolagani, rthallam

Subscribers: rthallam, ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D47354

Author: yusong-yan

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

@@ -222,14 +222,13 @@ TEST_F(PgRowLockTest, SelectForKeyShareWithRestart) {
     const auto table = "foo";
     auto conn = ASSERT_RESULT(Connect());
 
+    ASSERT_OK(conn.ExecuteFormat("DROP TABLE IF EXISTS $0", table));
     ASSERT_OK(conn.ExecuteFormat("CREATE TABLE $0(k INT, v INT)", table));
     ASSERT_OK(conn.ExecuteFormat("INSERT INTO $0 SELECT generate_series(1, 100), 1", table));
     ASSERT_OK(cluster_->FlushTablets());
 
     ASSERT_OK(conn.StartTransaction(IsolationLevel::SNAPSHOT_ISOLATION));
     ASSERT_OK(conn.FetchFormat("SELECT * FROM $0 WHERE k=1 FOR KEY SHARE", table));
-
-    ASSERT_OK(conn.ExecuteFormat("DROP TABLE $0", table));
   });
 }