Fix InitialExercisePairResult schema validation error

- Change quizzes array from .length(2) to .min(0).max(2)
- Allows empty array when errors occur during exercise generation
- Fixes 'Array must contain exactly 2 element(s)' validation error
- Error handling in quizSlice properly validates quiz count

This resolves the French language generation error and similar issues
where the API returns errors instead of exercise pairs.

Author: rgilks

app/domain/schemas.ts

@@ -149,7 +149,7 @@ export const apiResponseSchema = z.object({
 export type ApiResponse = z.infer<typeof apiResponseSchema>;
 
 export const InitialExercisePairResultSchema = z.object({
-  quizzes: z.array(GenerateExerciseResultSchema).length(2),
+  quizzes: z.array(GenerateExerciseResultSchema).min(0).max(2),
   error: z.string().nullable(),
 });