fix(migrate): surface lost SQL migration finalizer errors (#1389)

[Yusufihsangorgel]

Fixes #1389.

SQL migrations run via an unnamed return, so the finalizer's error (tx.Commit / tx.Rollback / bun.Conn.Close) was assigned to a local retErr inside the deferred function after return had already evaluated the result. In Go, the return expression is copied to the (unnamed) result before deferred functions run, so those finalizer errors were silently dropped:

• On a successful .tx.up.sql, a Commit error was lost. With WithMarkAppliedOnSuccess(true) the migrator could then mark the migration applied even though the commit's outcome was uncertain.
• On failure, the Rollback error was lost.
• For non-transactional SQL migrations, a Conn.Close error was lost.

This switches the migration closure to a named return and assigns the finalizer error to it. The execution error keeps priority: when the body fails, its error is preserved and joined with any rollback/close error via errors.Join.

Tests (migrate/migration_finalizer_test.go) use a small database/sql driver whose transactions fail to commit (and optionally fail exec):

• a Commit error is now returned from the migration (previously nil)
• an execution error is still returned when the body fails