test: add unit tests for challenge distribution, ranked challenges, answer edge cases, and mock bot coverage

Covers plan section 4.1: pickChallenge 70/30 distribution, pickRankedChallenge never returns choices,
True/False auto-generation, special characters and long answers, all 16 challenge types produce valid
mock responses, and timeout/error answer verification. 113 → 125 tests.

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

server/src/engine/challenges.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import { pickChallenge, getAllChallengeTypes, getAnswerPool } from './challenges.js'
+import { pickChallenge, pickRankedChallenge, getAllChallengeTypes, getAnswerPool } from './challenges.js'
 
 describe('pickChallenge', () => {
   it('returns a valid challenge', () => {
@@ -66,6 +66,64 @@ describe('pickChallenge', () => {
     // With 50 tries, choices should appear in more than one order
     expect(orders.size).toBeGreaterThan(1)
   })
+
+  it('distribution: ~70% factual, ~30% creative over many picks', () => {
+    let factual = 0
+    let creative = 0
+    const runs = 1000
+    for (let i = 0; i < runs; i++) {
+      const c = pickChallenge(new Set(), null)
+      if (c.scoring === 'factual') factual++
+      else creative++
+    }
+    const factualPct = factual / runs
+    // Allow ±10% tolerance due to randomness
+    expect(factualPct).toBeGreaterThan(0.55)
+    expect(factualPct).toBeLessThan(0.85)
+  })
+
+  it('True/False auto-generation for boolean answers', () => {
+    // Pick many challenges, find ones with answers = ['true'] or ['false']
+    let foundTFWithChoices = false
+    for (let i = 0; i < 500; i++) {
+      const c = pickChallenge(new Set(), null)
+      if (c.answers?.length === 1 && ['true', 'false'].includes(c.answers[0].toLowerCase())) {
+        expect(c.choices).toBeTruthy()
+        expect(c.choices!.length).toBe(2)
+        expect(c.choices!.sort()).toEqual(['False', 'True'])
+        foundTFWithChoices = true
+      }
+    }
+    expect(foundTFWithChoices).toBe(true)
+  })
+})
+
+describe('pickRankedChallenge', () => {
+  it('never returns choices', () => {
+    for (let i = 0; i < 100; i++) {
+      const c = pickRankedChallenge(new Set())
+      expect(c.choices).toBeUndefined()
+    }
+  })
+
+  it('returns valid challenge structure', () => {
+    const c = pickRankedChallenge(new Set())
+    expect(c.type).toBeTruthy()
+    expect(c.prompt).toBeTruthy()
+    expect(c.timeout_ms).toBeGreaterThan(0)
+    expect(c.baseDamage).toBeGreaterThan(0)
+  })
+
+  it('avoids used types', () => {
+    const types = getAllChallengeTypes()
+    const used = new Set(types.slice(0, -1))
+    let gotRemaining = false
+    for (let i = 0; i < 50; i++) {
+      const c = pickRankedChallenge(used)
+      if (c.type === types[types.length - 1]) gotRemaining = true
+    }
+    expect(gotRemaining).toBe(true)
+  })
 })
 
 describe('getAllChallengeTypes', () => {