refactor: improve test structure for Bun compatibility

- Update all test files to use named mock variables instead of inline mocks
- Clear mock history in beforeEach instead of using mock.restore()
- Add default mock implementations stored at module level
- Document that tests should not use --randomize flag due to mock.module() limitations
- All 89 unit tests pass consistently without randomization

src/__tests__/auth-simple.test.ts

@@ -2,7 +2,7 @@ import { describe, it, expect, mock, beforeEach, MockedFunction } from 'bun:test
 import { getSession } from '@/lib/auth-simple'
 
 // Mock next/headers
-vi.mock('next/headers', () => ({
+mock.module('next/headers', () => ({
   cookies: mock(() => {}).mockResolvedValue({
     get: mock(() => {}).mockReturnValue({ name: 'better-auth.session_token', value: 'test-token' }),
   }),