线程池的拒绝策略不得使用`DiscardPolicy`

## Rule content
线程池的拒绝策略不得使用`DiscardPolicy`。

## Problem description
`DiscardPolicy`不会更新`Future`的任务状态，导致后续调用`Future.get`会无限阻塞。
## Advice
1. 使用`AbortPolicy`
2. Future.get设置超时时间

Start by locating the rule implementation that analyzes thread-pool rejection policies, then inspect how its existing tests are organized. Add coverage for DiscardPolicy and verify that the rule recommends AbortPolicy or a timeout for Future.get; done means the prohibited policy is detected as described in the issue.

Written by the indexing model from the issue text.