fix: resolve quiz score calculation and MongoDB validation errors

[nasseralkawari]

This PR fixes backend issues related to quiz submission, score calculation, and MongoDB validation errors.

Problem

Submitting a quiz resulted in the following error:

Failed to calculate score
Result validation failed: wrongAnswers and correctAnswers are required

Root Cause

• calculateScore did not return required fields (correctAnswers, wrongAnswers)
• /submit route passed incomplete data to MongoDB
• Result model validation failed due to missing fields

Fix Implemented

• Updated calculateScore to return:

  • score
  • total
  • correctAnswers
  • wrongAnswers

• Updated /submit route to:

  • validate input properly
  • normalize result data
  • include all required fields before saving

• ensured answers are stored correctly

Testing

• Generated quiz from frontend

• Answered all questions

• Submitted successfully

• Verified:

  • score is calculated correctly
  • result is saved in MongoDB
  • no validation errors

Impact

• Fixes critical backend bug blocking quiz submission
• Enables full quiz flow (Generate → Answer → Submit → Feedback → Dashboard)

Notes

• Improves data consistency between frontend and backend
• Ensures MongoDB schema validation passes correctly