Start or resume signing an agreement (business)

Composes (or resumes) the calling member's e-signature envelope and returns it, plus a one-time embedded signing session when one can be issued. With purpose msa it signs the business's live MSA document named by document_id. With purpose custody_contract it signs the custody contract named by contract_type; the platform resolves the contract's document itself, so document_id must be absent. A request whose fields do not match its purpose, including an msa request without document_id, answers 400 esign_purpose_invalid with param naming the field. Requires e-sign enrollment (400 esign_not_enrolled, which is also the answer for custody_contract wherever custody contracts are not signed by e-signature), and the signer's authority attestation and title (400 esign_authority_attestation_required). An envelope another member started answers 409 esign_signer_conflict without a session; resuming the caller's own envelope with a title other than the one already placed on its document answers 409 esign_title_conflict; a document the caller cannot address answers 404 document_not_found; an agreement that changed while signing was starting, or a custody contract the business already signed, answers 409 esign_source_changed; a new envelope while this month's envelope count is at or above the platform's hard cap answers 409 esign_envelope_cap_reached (nothing is created or sent; resuming an envelope already started is not refused); a paused deployment answers 503, and an enrolled business on a deployment where e-signature is unavailable answers 503.