Fix non-404 error responses incorrectly reported as Available

[elchintoyirov]

• Non-404 status codes (403 bot blocking, 429 rate limiting, 5xx server errors)
  were being mapped to Available by the status_code detection path, producing
  false negatives
• Added WAFStatusCodes list (starting with LinkedIn's 999 anti-bot code) so
  status-code-based bot detection is fingerprinted alongside body-text WAF patterns
• Empty response bodies in message-type detection now yield Unknown instead of
  falling through to Claimed