Skip auto-linking in very large text nodes

A large comment body was causing Regexp::TimeoutError in production.
The URI regexp isn't catastrophically backtracking — it's linear — but
with a long enough string it exceeds the 1s Regexp.timeout set by
Rails.

Skip scanning text nodes over 10KB, which is well beyond any
reasonable content for auto-linking and still keeps us many orders of
magnitued under the timeout on an unloaded machine.

Fixes FIZZY-Q4

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/helpers/html_helper_test.rb

@@ -125,6 +125,17 @@ class HtmlHelperTest < ActionView::TestCase
     end
   end
 
+  test "skip auto-linking in very large text nodes" do
+    url = "https://example.com"
+    large_text = "x" * 5_000 + " #{url} " + "y" * 5_000
+    input = "<p>#{large_text}</p>"
+
+    result = format_html(input)
+
+    assert_no_match(/<a/, result)
+    assert_includes result, url
+  end
+
   test "don't autolink content in excluded elements" do
     %w[ figcaption pre code ].each do |element|
       assert_equal_html \