formatApiResponseJson --fields envelope filtering

Trap: applying --fields to an API envelope looks harmless because status metadata is useful, but it prevents expected filtering of the API body and can break array-element selection. Fix: apply field filtering to response.body while preserving {status,statusText,body}; support nested dot notation and array elements without mutating the source, while retaining literal keys containing dots. Because configured jsonTransform bypasses output.ts's generic root filtering, non-envelope payloads such as --dry-run request previews must explicitly call filterFields at the root when fields are supplied.