fix(sqlalchemy): atomic decorator must preserve logic exceptions #571

Problem: the atomic decorator is supposed to be a unit-of-work boundary, but _handle_db_exception converts every non-SQLAlchemy exception to InternalError. A swallowed default nested failure also raises DatabaseTransactionError with no cause, so the caller loses the original logic error type and details.

Fix: map only SQLAlchemyError to Database*Error; re-raise BaseError and every other exception unchanged. Store the first exception from a nested savepoint=False failure and re-raise it at the outer boundary after the rollback, instead of raising DatabaseTransactionError. DatabaseTransactionError stays for genuine SQLAlchemy transaction failures. Savepoint mode already re-raises the inner error to its caller.

Checklist

• map only SQLAlchemy failures; re-raise logic exceptions unchanged
• store and re-raise the original nested failure at the outer boundary
• update atomic tests and docs
• release notes for 5.28.0