Convert a qualified lead into a client. Creates a new client record,
sets lead status to CLOSED with closingStatus SUCCESS, and links the
lead to the new client via clientId.

convert is an action, not a status — the lead status enum is
[NEW, CONNECTED, PROCESSING, CLOSED] and does not include converted.

Requirements:

• closingStatus must be "SUCCESS"
• closingReasonId is optional (use GET /config/attributes?type=closingReason for options)
• All client data is extracted from the lead (clientFn, clientLn, cpn, cdob, etc.)