[Fix] Fix XML detection overriding HTML in content type detection

[Fix] Fix XML detection overriding HTML in content type detection

HTML parts containing an embedded <?xml?> prolog (e.g. inside the body)
were misdetected as application/xml. Since xml type has no_text=true,
this excluded the HTML part from text_parts entirely, causing
has_only_html_part() to return false and MIME_HTML_ONLY to not fire.

- Remove xml binary pattern from patterns.lua (it preempted the text
  heuristic which correctly prioritises HTML)
- Guard xml text pattern so it cannot override HTML when HTML markers
  are also present
- Raise xml text pattern weight to 40 to preserve pure XML detection