fix(api): normalize Stripe checkout request body

[dirtybits]

Summary

• normalize a literal JSON null checkout request body to the route's existing missing-skillId 400 contract
• retain the malformed-JSON 400 response
• add a regression proving literal null stops before rate-limit, database, or Stripe-session work

Test plan

• npm test --workspace @agentvouch/web -- __tests__/api/stripe-routes.test.ts --maxWorkers=1 --no-fileParallelism (52 tests)
• npm run format:check
• npm run lint:web
• npm run typecheck
• npm test --workspace @agentvouch/web -- --maxWorkers=1 --no-fileParallelism (128 files / 928 tests)
• npm exec --workspace @agentvouch/web -- next build --webpack
• git diff --check

Notes

No live Stripe checkout, database, wallet, or deployment flow was run. The production webpack build passed with the repository's existing ox/viem dynamic-dependency warning and expected static-generation fallback logs because local DATABASE_URL is absent.