fix: add validation for session questions and normalize user ID access across controllers

[rajdhakad9826]

📝 Pull Request Description

Related Issue

Closes #425

Summary

• Added request validation in addQuestionToSession to ensure a valid sessionId and a non-empty questions array are provided before processing the request.
• Added a null check in togglePinQuestion to return 404 Not Found when the requested question does not exist instead of triggering an internal server error.
• Standardized authenticated user ID access by replacing req.user.id and req.user._id || req.user.id with req.user._id in the affected controllers.

---

Type of Change

🐛 Bug fix (non-breaking change which fixes an issue)

[KaranUnique]

@rajdhakad9826 cloud you please resolve the conflicts

[rajdhakad9826]

@KaranUnique resolved