Update Submerchant

Merge-update a sub-merchant's draft onboarding form with a Phase-1 field subset. Only fields present in the body are changed — previously-set fields (from this or an earlier call) are preserved, except that websiteUrls/developmentUrls are whole-array replacements: providing a shorter array than a previous call clears the now-unused trailing discrete fields rather than leaving them stale. Rejects once the sub-merchant's application has been submitted for underwriting review.