fix(web_fetch): replace hand-written HTML parser with html2text/htmd

- Replace fragile hand-written html_to_text() and html_to_markdown() with
  html2text (v0.16) and htmd (v0.5) library calls
- Fixes UTF-8 multi-byte character panic in html_to_text()
- Add test for multi-byte UTF-8 content (Diseño, café, résumé)
- Removes ~80 lines of brittle parsing code

Author: ZhiXiao-Lin

Cargo.lock

@@ -68,6 +68,10 @@ sha256 = "1.5"
 # Shell argument parsing
 shell-words = "1.1"
 
+# HTML to text/markdown conversion
+html2text = "0.16"
+htmd = "0.5"
+
 # Base64 for image encoding
 base64 = "0.21"