Stripe Projects tests need Stripe customer mock after billing auto-provisioning

Stripe Projects test patterns: (1) Control tests with cell models: assume_test_silo_mode(SiloMode.REGION) + @control_silo_test(cells=create_test_cells('us')). (2) Mock Stripe customer: standalone_subscription_options creates customer via outbox — tests need @responses.activate with POST https://api.stripe.com/v1/customers returning {"id": "cus_test_xxx", "object": "customer"} before org creation. (3) HMAC: dev.py uses different secret — test classes using sign_request need @override_settings(STRIPE_PROJECTS_HMAC_SECRET=HMAC_SECRET).