Cleanup Tax lines in case of a failed cart completion

Editor assessment

The subject states the defect remedy exactly: tax lines are now cleaned up when cart completion fails, preventing orphaned tax records after a failed checkout. Condition, entity, and consequence are all recoverable without a diff. Fix is right and the medusa scope accurate. No body explains the leak's mechanism, which the headline largely implies, and the capitalised 'Cleanup' is cosmetic. Precise, well-scoped, and self-sufficient. A strong, publishable fix message.