Add Service Catalog end-to-end test script

Tests the full SC product → CloudFormation → Lambda → GitHub stack:
  1. AWS credentials check
  2. SC product + artifact lookup
  3. Provision product (launch CFN stack via SC)
  4. Poll CFN stack events to terminal state
  5. GitHub repo/PR verification
  6. Auto-terminate provisioned product (--no-cleanup to skip)

Usage:
  python scripts/test_service_catalog.py [REPO_NAME]
  python scripts/test_service_catalog.py --no-cleanup [REPO_NAME]
  python scripts/test_service_catalog.py --terminate PRODUCT_NAME
  python scripts/test_service_catalog.py --timeout 900 [REPO_NAME]