Generic, registry-driven metadata retrieve: accepts a batch of components of any SDR metadata type and returns each in the same shape `deploy-metadata-xml` accepts (`xml` vs `xml`+ `content` vs `files`), so a retrieved component can be fed straight back into deploy with no translation. See `sdr_retrieve.ts` for the per-type response shape and decomposed-child handling (e.g. `CustomField`/`ValidationRule` come back as a standalone document, not the whole parent `CustomObject`). Unlike deploy, malformed requests are narrower: only an unknown type or a duplicate type+fullName pair throws `BadRequestException`. A component the org didn't return (wrong name, doesn't exist) comes back `success: false` on that component, not as an HTTP error.