fix: add mandatory field validation for question and answer in createFlashcard (#1198)

[suhaniiz]

📝 Pull Request Description

Related Issue

Closes #1198

Summary

Added early payload validation in createFlashcard to check for missing, empty, or non-string question and answer fields before processing. Previously, invalid or whitespace-only inputs bypassed controller checks and led to unhandled Mongoose schema errors or malformed entries. The endpoint now returns an explicit 400 Bad Request and sanitizes valid inputs using .trim().

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Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature
• ♻️ Refactoring
• 📝 Documentation update
• 🎨 UI/UX improvement
• 🔥 Other(please describe) ______

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How Has This Been Tested?

Describe the testing steps performed.

• Sent POST /api/flashcards with an empty body {} and verified a 400 Bad Request response.
• Sent payloads with whitespace strings like {"question": " ", "answer": ""} and confirmed a 400 Bad Request response.
• Sent valid string payloads and verified 201 Created with trimmed input fields.

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Screenshots (if applicable)

N/A

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Checklist

• My code follows the project's guidelines
• I have tested my changes
• I have updated documentation where necessary
• I have linked the related issue
• My changes do not introduce new warnings or errors

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📥 Commits

Reviewing files that changed from the base of the PR and between 4ba01aa and 2db631a.

📒 Files selected for processing (1)

• backend/controllers/flashcardController.js

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