Defang generate with Redis should automatically add x-defang-redis

I generated a project with defang generate which ended up looking like this:

services:
  service1:
    restart: always
    build:
      context: .
      dockerfile: Dockerfile
    environment:
      - REDIS_HOST=redis
      - REDIS_PORT=6379

  redis:
    image: redis:alpine
    ports:
      - 6379:6379

Good starting point, but we should make sure that generated code includes x-defang-redis: true if it includes Redis at all.

Start at the defang generate entry point and trace how the generated Compose configuration is assembled and how Redis services are identified. Update the generated output so any project containing Redis includes x-defang-redis: true, then run the relevant generation checks or compare generated output with the example in this issue.