Restore correctChoiceIndex to MCQ practice tests

[google-labs-jules]

The user requested to restore the correctChoiceIndex field in the practice test MCQ records because its removal was premature and caused data loss potential.

Key changes:

1. Database Migration: Edited db/migrations/202606252121_update_practice_test_questions.sql to stop removing correctChoiceIndex in the up step. Enhanced the down step to use correctChoiceIndex and answeredIndex to correctly rebuild the answeredTerm object from the distractors and the correct term.
2. GraphQL Schema: Added correctChoiceIndex: Int! to both MCQ (query) and MCQInput (mutation) types in graph/query.graphqls and graph/mutation.graphqls.
3. Go Code Generation: Ran go generate ./... to update graph/model/models_gen.go and other generated files.
4. Tests: Updated tests/practice_test_test.go to include correctChoiceIndex in the test cases, ensuring the API correctly accepts and processes the new field.

The changes ensure that the MCQ structure correctly reflects the intended logic: the term is the correct answer, distractors are the wrong answers, and correctChoiceIndex specifies where to insert the correct answer into the array for display. answeredIndex remains as the user's selected index in that combined array.

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PR created automatically by Jules for task 15426658880331694306 started by @ehanahamed

[google-labs-jules]

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