input: Write a python function to sanitize a string by removing HTML tags, special characters, and leading/trailing whitespace, converting ASCII characters to Unicode, and converting the string to lowercase.
  assert sanitize_string('  Hello <b>World</b>! &#1234;  ') == 'hello world \\u04d2'
  assert sanitize_string('<div>Test & Test!</div>') == 'test  test'
  assert sanitize_string('Sample 123 &amp; <test>!') == 'sample 123  test'
output: def sanitize_string(input_string):
      import re
      from html import unescape
  
      # Remove HTML tags
      sanitized = re.sub(r'<.*?>', '', input_string)
      # Unescape HTML entities
      sanitized = unescape(sanitized)
      # Remove special characters except for spaces
      sanitized = re.sub(r'[^A-Za-z0-9 ]+', '', sanitized)
      # Convert to lowercase
      sanitized = sanitized.lower()
      # Strip leading/trailing whitespace
      sanitized = sanitized.strip()
      # Convert ASCII to Unicode
      sanitized = sanitized.encode('unicode_escape').decode('utf-8')
      
      return sanitized