Utf8SanitizationTest:
  ✓ sanitizeUtf8 preserves valid UTF-8 strings
  ✓ sanitizeUtf8 handles arrays correctly
  ✓ sanitizeUtf8 handles nested arrays
  ✓ Malformed UTF-8 characters are sanitized and JSON-encodable
  ✓ Control characters are removed
  ✓ Text with control characters is JSON-encodable after sanitization
  ✓ Complex API response with malformed UTF-8 is sanitized and encodable
  ✓ Sanitization preserves data structure
  ✓ sendResponse sanitization prevents JSON encoding errors