Allowed Scopes: experimental:chart-of-accounts:write

Partially updates one or more existing accounts by id. Validation is all-or-nothing: if any item has no field to update (400) or any id is not found (404), the request fails and nothing is written. Once validated, each account is patched independently: successful updates are returned in accounts, and per-item failures are returned in failed so callers can retry only the failed ids. Use PUT /v1/experimental/accounts for bulk upsert by type and PUT /v1/experimental/accounts/set-purpose to assign account purposes.