Make replication user creation duplicate-safe

[dati18]

Bug: T414813

[tarrow]

Seems like a likely cause of the issue. I was able to replicate it by
docker compose down -v
docker compose up -d
docker compose down
docker compose up -d

and saw in the logs:

mysql-replica-1  | 2026-01-23 16:37:34 16 [Note] Slave SQL thread initialized, starting replication in log 'mysql-bin.000002' at position 342, relay log './mysqld-relay-bin.000001' position: 4
mysql-replica-1  | 2026-01-23 16:37:34 15 [Note] Slave I/O thread: connected to master 'repl@mysql:3306',replication started in log 'mysql-bin.000002' at position 342
mysql-replica-1  | 2026-01-23 16:37:34 16 [ERROR] Slave SQL: Error 'Operation CREATE USER failed for 'repl'@'%'' on query. Default database: ''. Query: 'CREATE USER 'repl'@'%'', Gtid 0-1-186, Internal MariaDB error code: 1396
mysql-replica-1  | 2026-01-23 16:37:34 16 [Warning] Slave: Operation CREATE USER failed for 'repl'@'%' Error_code: 1396
mysql-replica-1  | 2026-01-23 16:37:34 16 [ERROR] Error running query, slave SQL thread aborted. Fix the problem, and restart the slave SQL thread with "SLAVE START". We stopped at log 'mysql-bin.000002' position 342
mysql-replica-1  | 2026-01-23 16:37:34 16 [Note] Slave SQL thread exiting, replication stopped in log 'mysql-bin.000002' at position 342
mysql-replica-1  | 2026-01-23 16:37:34 16 [Note] master was mysql:3306

which no longer happens after checking out this PR