Partial update. Keys omitted from the request body are left unchanged; send a key with a
null, empty, or whitespace-only value to clear it. Prefer this over
update-customer-metadata when setting or clearing individual keys, as it cannot disturb
keys you did not name. Archived keys may only be cleared, never changed. Every key must
name a definition assigned to customers: an unknown key, or one assigned only to another
entity type, fails the request with a 400. Keys are never silently ignored. The write is
atomic: if any key fails validation, nothing is written.