feat(flow): 添加驳回实例删除功能并优化代码结构

- 在HrmFlowInstanceController中新增deleteRejected接口用于删除驳回状态的流程实例
- 在HrmFlowInstanceService中实现deleteRejectedInstance方法，包含状态校验和级联删除逻辑
- 修复代码中的缩进格式问题，统一代码风格
- 优化HrmFlowTaskServiceImpl中查询待办任务的逻辑，避免空指针异常
- 更新注释中的引号格式，保持代码一致性
- 添加数据验证注解确保参数合法性

fad-hrm/src/main/java/com/ruoyi/hrm/controller/HrmFlowInstanceController.java

@@ -79,4 +79,10 @@ public class HrmFlowInstanceController extends BaseController {
     public R<Void> revoke(@PathVariable @NotNull Long instId) {
         return toAjax(service.revokeInstance(instId));
     }
+
+    @Log(title = "删除驳回实例", businessType = BusinessType.DELETE)
+    @DeleteMapping("/deleteRejected/{instId}")
+    public R<Void> deleteRejectedInstance(@PathVariable @NotNull Long instId) {
+        return toAjax(service.deleteRejectedInstance(instId));
+    }
 }