fix(fe): fix user testcase form initialization bug

[eunnbi]

Description

모달 닫힐 때 form을 기본값으로 초기화합니다.

https://github.com/user-attachments/assets/831a996b-3c3a-40f4-a332-c4c30c728317

closes TAS-1069

Additional context

---

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines
• [x] Read the Contributing Guidelines and follow the Commit Convention
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• [ ] Ideally, include relevant tests that fail without this PR but pass with it.

[notion-workspace[bot]]

작업24-2 2차 배포 QA) User Testcase 추가 모달에서 새로운 testcase를 작성한 뒤에 add 버튼을 누르지 않고 cancel 버튼을 눌렀을 때 생기는 오류