Previously when PG_REUSE=1 and PG_VERSION existed but the target database had
never been successfully created (e.g. devshell exited mid-init in a prior run),
postgres-init skipped createdb and the subsequent psql connection failed with
'database "<db>" does not exist'.
Now on reuse path we probe pg_database and create the target DB if missing,
making postgres-init fully idempotent across stale-state recovery.
Adds postgres-init-reuse-missing-db test.
