feat(backend): implement getSubmissionById

[r4ai]

close #133

Summary

This pull request includes significant updates to the backend/src/api/paths/submissions.ts file, focusing on improving type validation and enhancing database query handling for submission data retrieval.

Type Validation Improvements:

• Updated the SubmissionIdParam schema to validate submissionId as a string that is coerced into a nonnegative integer. This change ensures better type handling and consistency.

Database Query Enhancements:

• Modified the getSubmissionByIdRoute to perform an actual database query using Prisma. The new implementation includes fetching related result and testResults data, ensuring comprehensive data retrieval.
• Added error handling for invalid submission and test statuses, throwing errors if the statuses are not valid. This ensures data integrity and prevents unexpected states.
• Included a detailed response structure for the submission data, ensuring all necessary fields are returned in the JSON response. This includes code, id, language, problem_id, result, student_id, submitted_at, and test_results.

[sou1118]

LGTM!