Fetches the signed eSign document for a docket. Once every signer has signed, the response carries the completed PDF as base64 (signing_status: "signed", content populated). While one or more signers are still pending, the same HTTP 200 is returned with signing_status: "pending", document_id: null and content: null, so branch on signing_status, not on status. Pass an optional document_id to fetch a specific document when the docket has more than one. Requires X-Client-ID and X-API-KEY headers.