Verified against the live Helcim v2 API during the deploy migration:
- POST /payment-plans requires { paymentPlans: [plan] } wrapper (mirrors
the POST /subscriptions shape), and response is { data: [plan] }.
- taxType 'customer' rejects as ERR_VALIDATION_FAILED; must be 'no_tax'
with taxCalculation 'country_province'.
- termLength:1 rejects when termType:'forever' — drop the field.
- GET /subscriptions returns an empty body (not JSON) when no subs exist;
tolerate that instead of failing with 'Unexpected end of JSON input'.
Plans created in the Helcim account: Monthly=50302, Annual=50303.
