Fix Redis delayed message promotion

[iamdinhthuan]

Fixes #431.

Redis delayed messages were promoted by enqueueing a copy to the canonical queue and then acking the original delay-queue message as two separate operations. If the original delay message was requeued before the ack completed, the same logical message could be promoted more than once.

This PR adds a Redis-specific consumer hook that promotes due delayed messages atomically in the Redis Lua dispatch script. The generic worker path still falls back to the existing enqueue-then-ack behavior for consumers that do not implement the hook.

Verification:

• .venv/bin/python -m pytest tests/test_worker.py::test_delayed_messages_use_consumer_promotion_hook tests/test_redis.py::test_redis_consumer_promotes_delayed_messages_atomically -q
• .venv/bin/python -m pytest tests/test_redis.py tests/test_worker.py -q with Redis 7 running locally via Docker: 23 passed in 66.07s
• .venv/bin/python -m black --check dramatiq/broker.py dramatiq/worker.py dramatiq/brokers/redis.py tests/test_worker.py tests/test_redis.py
• .venv/bin/python -m isort -c dramatiq/broker.py dramatiq/worker.py dramatiq/brokers/redis.py tests/test_worker.py tests/test_redis.py
• .venv/bin/python -m flake8 dramatiq/broker.py dramatiq/worker.py dramatiq/brokers/redis.py tests/test_worker.py tests/test_redis.py
• .venv/bin/python -m mypy dramatiq/broker.py dramatiq/worker.py dramatiq/brokers/redis.py tests/test_worker.py tests/test_redis.py
• Standalone fakeredis smoke check for the new Lua command.