OPDC API error code

CHANGE_NUMBER_REQUIRED

HTTP status 422

No change number was sent and the job's last one is not a number Sage's NEXT can increment. 422. Nothing is written.

The API answers with a problem document (RFC 9457) whose type is https://twinn.engineering/errors/change-number-required, and with X-Error-Code: CHANGE_NUMBER_REQUIRED. Branch on the code, never on the detail text.

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