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-account-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 accounts: 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.