Voids an in-progress document. Works for both single signature PDFs (documentType=PDF) and PDF templates (documentType=PDF_TEMPLATE). Idempotent: already-voided documents return 200 with Document is already voided.

For a single signature PDF, send the public documentId UUID (documentType defaults to PDF). For a PDF template, send the template UUID as documentId, set documentType to PDF_TEMPLATE, and pass the respondent Mongo ObjectId (createdDocumentId from /pdf-template-lambda) as responseId.

Documents in SIGNED, PAID, DECLINED, EXPIRED, or DELETED cannot be voided.