fix: deduplicate distractors, replace tautological test

Problem

Two issues in createGameSession and its test suite:

If getDistractors returned the correct answer as one of the distractors, createGameSession would include it in the options array without filtering it out. indexOf would then find the first occurrence, which might not be the one intended as the correct answer — producing a question where the correct answer appears twice and the stored correctOptionId is wrong.
The test "distractors are never the correct answer" was tautological — it filtered the correct answer out of the options array, then asserted the remaining items were not the correct answer. It was testing that Array.filter() works. It could never fail.

Options considered

Option A — Filter duplicates after fetching, request extra distractors as buffer ✅

Filter out any distractor that matches the correct answer after fetching. Request 6 distractors instead of 3 to ensure enough remain after deduplication. Take the first 3 valid ones with slice(0, 3).

Chosen because: deduplication at the service layer is the right place — getDistractors shouldn't need to know what the correct answer is. Requesting extra provides a buffer against collisions.

Option B — Fix `getDistractors` to never return the correct answer

Add a NOT filter in the database query.

Not chosen for this ticket — the database query is in @lila/db and is a separate concern. The service layer should be defensive regardless of what the model layer returns.

Solution

Filter distractors against the correct answer before building options:

const uniqueDistractors = distractorTexts.filter(
  (t) => t !== term.targetText,
);
const optionTexts = [term.targetText, ...uniqueDistractors.slice(0, 3)];

Request 6 distractors instead of 3 to account for potential duplicates

Replaced tautological test with a test that actually exercises the duplicate case:

it("correct answer appears exactly once even if getDistractors returns a duplicate", ...)

Added distractor failure propagation test:

it("propagates getDistractors failure", ...)

Files changed

apps/api/src/services/gameService.ts — deduplication logic, distractor count increased to 6
apps/api/src/services/gameService.test.ts — tautological test replaced, failure test added

Commit

fix: deduplicate distractors, replace tautological test, add distractor failure test

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