fix : added max length guard to achievement unlock array items (issue #1404)

[tmdeveloper007]

Summary of What Has Been Done

Added a .max(100) guard to the unlockedAchievements array item strings in savedAchievementsSchema in backend/Input_validators/ValidateAchievement.js.

Changes Made

• Modified backend/Input_validators/ValidateAchievement.js: Added .max(100, "Achievement ID must be at most 100 characters") to the achievement ID string
• Added backend/tests/achievementValidator.maxLength.unit.test.js: 6 unit tests covering max-length boundary cases

Impact it Made

• Adds defense-in-depth against malformed or malicious payloads
• Ensures consistency even if achievement ID generation logic changes in the future
• Minimal change with no breaking behavior for valid inputs

Closes #1404

Note: Please assign this PR to the tmdeveloper007 account.

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📥 Commits

Reviewing files that changed from the base of the PR and between 8acb5b8 and 7aefe43.

📒 Files selected for processing (8)

• backend/Input_validators/ValidateAchievement.js
• backend/Input_validators/ValidateFlashcard.js
• backend/Input_validators/ValidateQuestions.js
• backend/Input_validators/ValidateSession.js
• backend/tests/achievementValidator.maxLength.unit.test.js
• backend/tests/flashcardValidator.maxLength.unit.test.js
• backend/tests/questionValidator.maxLength.unit.test.js
• backend/tests/sessionValidator.maxLength.unit.test.js

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