✨ List round 回傳該 Round 所有 Challenge

add challenge_list field

描述 Description

這支 PR 所做的事情，盡可能詳細取代本文字。 Replace this text and describe This PR doing(Better to be detail)

GET: /api/round/ 取的所有 is_valid round 並且回傳

[
 {
   "id":1,
   "challenge_list": [],
   "start_time":"",
   "end_time":"",
   "is_valid":true,
 }
]

相關的 Issues #(ticket number) / 無 Related Issues #(ticket number) / None

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update

~~- [ ] 我已經在本地手動測試過，確保功能的完整性和穩定性 I have done fully correct manual test~~