Fix handling of malformed/unusual HTML (#34201)

app/lib/plain_text_formatter.rb

@@ -16,7 +16,15 @@ class PlainTextFormatter
     if local?
       text
     else
-      node = Nokogiri::HTML5.fragment(insert_newlines)
+      begin
+        node = Nokogiri::HTML5.fragment(insert_newlines)
+      rescue ArgumentError
+        # This can happen if one of the Nokogumbo limits is encountered
+        # Unfortunately, it does not use a more precise error class
+        # nor allows more graceful handling
+        return ''
+      end
+
       # Elements that are entirely removed with our Sanitize config
       node.xpath('.//iframe|.//math|.//noembed|.//noframes|.//noscript|.//plaintext|.//script|.//style|.//svg|.//xmp').remove
       node.text.chomp