The defer closure checked a local err variable for commit/rollback
decisions, but err remained nil after a successful BeginTx. When
txBody failed, the error was returned directly without assigning to
err, so the defer always committed instead of rolling back.
Additionally, since err was not a named return value, the defer's
Commit error assignment was silently swallowed.
Replace the error-prone defer pattern with explicit rollback on
txBody failure and a direct Commit return.
