Migrate Return service to ts

Editor assessment

A TypeScript migration wearing a feat label. Converting the Return service changes no behavior and ships no capability, so refactor(medusa) is the honest classification; as written, changelog automation will list this among Features and infer a minor bump that misleads users scanning release notes for new functionality. The subject is otherwise serviceable — imperative, concise, PR referenced — though 'to ts' lowercases the abbreviation, where writing 'TS' or 'Typescript' would keep the header grep-able across the migration series. No body, which is acceptable for mechanical conversions. No breaking markers, correctly absent. Teams should codify one type for TS conversions to prevent exactly this ambiguity; the message is fine, the type is not.