fix: add cross-validation for questions/user_answers/correct_answers list lengths

[ravimahatonp]

Summary

Closes #7

Problem

GenerateFeedbackRequest accepts questions, user_answers, and correct_answers as independent lists with no cross-validation on their lengths. In feedback_engine.py, zip(user_answers, correct_answers) silently truncates to the shorter list, causing answers to be silently dropped and incorrect score/feedback computation.

Solution

• Added a @model_validator(mode="after") to GenerateFeedbackRequest that checks len(questions) == len(user_answers) == len(correct_answers).
• Mismatched lengths now return a clear 422 Unprocessable Entity error with the exact lengths reported (e.g., questions=3, user_answers=1, correct_answers=3).
• Added 8 targeted unit tests covering matching lengths, each mismatch direction, empty lists, and error message content.

Test Results

All 41 tests pass (33 existing + 8 new), zero regressions.