Actualizar plan

PUT https://api.geopagos.io/api/v1.0/plans/{planId}

Body — todos los campos opcionales

name, description, amount, currency, billing_frequency, billing_cycles, trial_period_days, status ("active"|"inactive"), redirect_url, external_reference, merchant_name, merchant_fiscal_id.

Response 200 — mismo esquema que GET /plans/{planId}.