Configure Prefect 3 task result caching with cache_policy and cache_key_fn

Steps

1. Set cache_policy on a @task — built-ins include DEFAULT (inputs + code + flow run ID), INPUTS, TASK_SOURCE, FLOW_PARAMETERS, and NO_CACHE, and policies can be composed with + and -.
2. For custom cache logic, pass cache_key_fn(context, parameters) instead of, or alongside, a built-in policy to compute a custom cache key from task inputs.
3. Set cache_expiration to a timedelta to bound how long a cached result stays valid, rather than letting cache hits persist indefinitely.
4. Confirm result persistence is actually enabled — caching in Prefect 3 requires persisted results, which is off by default and controlled by PREFECT_RESULTS_PERSIST_BY_DEFAULT.
5. Avoid persist_result=False on any task you want cached — it disables caching regardless of the configured cache_policy, since there's nothing to reuse.