test: preserve legacy subscription billing rail

Amp-Thread-ID: https://ampcode.com/threads/T-019fbba1-3994-7630-a20c-19b9965b4fd2
Co-authored-by: Amp <amp@ampcode.com>

Author: huntcsg

browser_tests/fixtures/helpers/SubscriptionHelper.ts

@@ -49,6 +49,7 @@ function toBillingStatus(
   return {
     is_active: status.is_active ?? false,
     has_funds: status.has_fund ?? false,
+    billing_rail: 'legacy_stripe',
     ...(status.subscription_tier
       ? { subscription_tier: status.subscription_tier }
       : {}),