docs(be): add JSDoc into AssignmentProblemService

[nhjbest22]

Description

AssignmentProblemService의 함수들의 역할을 명확히하기 위해 JSDoc을 추가했습니다.

Additional context

• 추가적인 리팩토링을 수행하였습니다.

1. 불필요한 Return Type 지정을 삭제하였습니다.
   다음을 참고하였습니다. 불필요한 Type을 피해요.
   
2. return시에 사용되는 불필요한 await 문을 삭제하였습니다.
3. select를 사용해 실제 사용되는 column만을 가져올 수 있도록 하였습니다.

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Closes TAS-2441

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

• Read the Contributing Guidelines
• Read the Contributing Guidelines and follow the Commit Convention
• 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.

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[notion-task-integration]

Admin Assignment-problem JSDoc 추가