Editing a model in the Admin UI (e.g. to change credential) unconditionally
included input_cost_per_token: 0 and output_cost_per_token: 0 in the PATCH
payload, overriding built-in pricing from model_prices_and_context_window.json.
Guard cost fields with form.isFieldTouched so they are only sent when the
user explicitly modifies them. Intentional $0 cost (for budget bypass) still
works because the guard checks touched + non-null, not non-zero.