feat: add post-answer explanations and improve results reliability

[PiyushSharma-05]

Overview

This PR improves the post-session review experience by displaying contextual explanations for both teachers and students while addressing backend issues affecting result generation and question processing.

---

Features

Post-Answer Explanations

• Added contextual explanations to the results page, allowing teachers and students to review the reasoning behind each answer.
• Students now see explanations for incorrect or missed questions, while teachers can review explanations alongside the correct answers.
• Included graceful fallbacks when explanations are unavailable.

Backend Improvements

• Added backend support to ensure question explanations are available throughout the results workflow.
• Fixed an issue affecting session summary generation.
• Improved resilience when processing incomplete or null AI-generated responses.

---

Result

This update provides richer learning feedback after each session while improving the stability and reliability of the assessment workflow.