Fix schema validation for correctAnswer field

- Update ExerciseContentSchema to use z.enum(['A', 'B', 'C', 'D']) for correctAnswer
- Update QuizDataSchema to use z.enum(['A', 'B', 'C', 'D']) for correctAnswer
- This ensures correctAnswer can only be 'A', 'B', 'C', or 'D' as expected
- Should fix the answer validation issues where correct answers were marked incorrect

Author: rgilks

app/domain/schemas.ts

@@ -34,7 +34,7 @@ export const QuizDataSchema = z.object({
   question: z.string(),
   options: z.object({ A: z.string(), B: z.string(), C: z.string(), D: z.string() }),
   correctExplanation: z.string().optional().nullable(),
-  correctAnswer: z.string().optional().nullable(),
+  correctAnswer: z.enum(['A', 'B', 'C', 'D']).optional().nullable(),
   allExplanations: z
     .object({ A: z.string(), B: z.string(), C: z.string(), D: z.string() })
     .optional()
@@ -85,7 +85,7 @@ export const ExerciseContentSchema = z.object({
   paragraph: z.string(),
   question: z.string(),
   options: z.object({ A: z.string(), B: z.string(), C: z.string(), D: z.string() }),
-  correctAnswer: z.string(),
+  correctAnswer: z.enum(['A', 'B', 'C', 'D']),
   allExplanations: z.object({
     A: z.string(),
     B: z.string(),