fix: update tests for creative MC and flaky mock answer checks

Creative challenges now have auto-generated MC choices with correct
answers. Mock bad answers can be empty at any elo, so test checks
proportion instead of requiring all non-empty.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/mock.test.ts

@@ -74,16 +74,21 @@ describe('mockResponse', () => {
     expect(highAvg).toBeLessThan(lowAvg)
   })
 
-  it('creative challenges return non-empty answers', () => {
+  it('creative challenges return answers (most non-empty at high elo)', () => {
+    let nonEmpty = 0
+    let total = 0
     for (let i = 0; i < 50; i++) {
       const challenge = pickChallenge(new Set(), null)
       if (challenge.scoring !== 'creative') continue
 
-      const resp = mockResponse(challenge, 'witty', 1500)
+      const resp = mockResponse(challenge, 'witty', 1800)
       if (!resp.timedOut && !resp.error) {
-        expect(resp.answer.length).toBeGreaterThan(0)
+        total++
+        if (resp.answer.length > 0) nonEmpty++
       }
     }
+    // At elo 1800, bad answer chance is very low; most should be non-empty
+    expect(nonEmpty).toBeGreaterThan(total * 0.7)
   })
 
   it('trash talk is always a string', () => {