Update customer KYC information

A flat map of KYC document types to their values. Send the document map directly as the request body (do not wrap it in a kyc field). Each key must be one of the allowed KYC document types and each value is a string.

PHONE and COUNTRY are not accepted here; they are profile fields owned by PATCH /api/v1/customer/{customerId}. BVN is not accepted either; it is owned by POST /api/v1/customer/{customerId}/verify, which writes it only after a successful bank verification. Sending any of the three returns 400 INVALID_KYC_DOCUMENT_TYPE, as does an empty body.

Value rules: DATE_OF_BIRTH must be a valid YYYY-MM-DD date for a customer aged 13 to 75; every other value must be at least 2 characters. A value that breaks a rule returns 400 INVALID_KYC_DOCUMENT_TYPE.