test: add mock and arena tests — 21 more tests (86 total)

- mock.test.ts: 7 tests for ELO-weighted response quality, failure rates, timing
- arenas.test.ts: 14 tests for arena data integrity, pickArena, randomArena

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

server/src/engine/arenas.test.ts

@@ -0,0 +1,105 @@
+import { describe, it, expect } from 'vitest'
+import { ARENAS, pickArena, randomArena } from './arenas.js'
+
+describe('ARENAS', () => {
+  it('has 25 arenas', () => {
+    expect(ARENAS.length).toBe(25)
+  })
+
+  it('all arenas have required fields', () => {
+    for (const arena of ARENAS) {
+      expect(arena.id).toBeTruthy()
+      expect(arena.name).toBeTruthy()
+      expect(arena.description).toBeTruthy()
+      expect(typeof arena.id).toBe('string')
+      expect(typeof arena.name).toBe('string')
+      expect(typeof arena.description).toBe('string')
+    }
+  })
+
+  it('all arena IDs are unique', () => {
+    const ids = ARENAS.map(a => a.id)
+    expect(new Set(ids).size).toBe(ids.length)
+  })
+
+  it('all arena names are unique', () => {
+    const names = ARENAS.map(a => a.name)
+    expect(new Set(names).size).toBe(names.length)
+  })
+
+  it('arenas with modifier also have modifierDescription', () => {
+    for (const arena of ARENAS) {
+      if (arena.modifier) {
+        expect(arena.modifierDescription).toBeTruthy()
+      }
+    }
+  })
+
+  it('arenas without modifier have null modifierDescription', () => {
+    for (const arena of ARENAS) {
+      if (!arena.modifier) {
+        expect(arena.modifierDescription).toBeNull()
+      }
+    }
+  })
+
+  it('known modifiers are present', () => {
+    const modifiers = ARENAS.map(a => a.modifier).filter(Boolean)
+    expect(modifiers).toContain('speed_2x')
+    expect(modifiers).toContain('roast_2x')
+    expect(modifiers).toContain('food_2x')
+    expect(modifiers).toContain('meme_2x')
+  })
+})
+
+describe('pickArena', () => {
+  it('same choices return The Singularity', () => {
+    const arena = pickArena(5, 5)
+    expect(arena.id).toBe('the_singularity')
+  })
+
+  it('different choices return a non-singularity arena', () => {
+    const arena = pickArena(3, 7)
+    expect(arena.id).not.toBe('the_singularity')
+  })
+
+  it('deterministic for same inputs', () => {
+    const a1 = pickArena(10, 20)
+    const a2 = pickArena(10, 20)
+    expect(a1.id).toBe(a2.id)
+  })
+
+  it('different inputs give different arenas (most of the time)', () => {
+    const arenas = new Set<string>()
+    for (let i = 0; i < 50; i++) {
+      arenas.add(pickArena(i, i + 1).id)
+    }
+    expect(arenas.size).toBeGreaterThan(3)
+  })
+})
+
+describe('randomArena', () => {
+  it('never returns The Singularity', () => {
+    for (let i = 0; i < 50; i++) {
+      const arena = randomArena()
+      expect(arena.id).not.toBe('the_singularity')
+    }
+  })
+
+  it('returns variety', () => {
+    const arenas = new Set<string>()
+    for (let i = 0; i < 100; i++) {
+      arenas.add(randomArena().id)
+    }
+    expect(arenas.size).toBeGreaterThan(5)
+  })
+
+  it('returns valid arena objects', () => {
+    for (let i = 0; i < 20; i++) {
+      const arena = randomArena()
+      expect(arena.id).toBeTruthy()
+      expect(arena.name).toBeTruthy()
+      expect(arena.description).toBeTruthy()
+    }
+  })
+})