Use prefix for TableOfContents filter hrefs

TableOfContents filter generates hrefs for each header in markdown,
if the header text consists from digits and redacted symbols only,
e.g. "123" or "1.0 then the auto-generated href has the same format
as issue references.

If the generated id contains only digits, then 'anchor-' prefix is
prepended to the id.

Closes #38473

changelogs/unreleased/anchor-issue-references.yml

+---
+title: Fix false positive issue references in merge requests caused by header anchor
+  links.
+merge_request:
+author:
+type: fixed

lib/banzai/filter/table_of_contents_filter.rb

@@ -32,6 +32,7 @@ module Banzai
               .gsub(PUNCTUATION_REGEXP, '') # remove punctuation
               .tr(' ', '-') # replace spaces with dash
               .squeeze('-') # replace multiple dashes with one
+              .gsub(/\A(\d+)\z/, 'anchor-\1') # digits-only hrefs conflict with issue refs
 
             uniq = headers[id] > 0 ? "-#{headers[id]}" : ''
             headers[id] += 1

spec/lib/banzai/filter/table_of_contents_filter_spec.rb

@@ -65,6 +65,13 @@ describe Banzai::Filter::TableOfContentsFilter do
         expect(doc.css('h2 a').first.attr('href')).to eq '#one-1'
       end
 
+      it 'prepends a prefix to digits-only ids' do
+        doc = filter(header(1, "123") + header(2, "1.0"))
+
+        expect(doc.css('h1 a').first.attr('href')).to eq '#anchor-123'
+        expect(doc.css('h2 a').first.attr('href')).to eq '#anchor-10'
+      end
+
       it 'supports Unicode' do
         doc = filter(header(1, '한글'))
         expect(doc.css('h1 a').first.attr('id')).to eq 'user-content-한글'

spec/lib/gitlab/reference_extractor_spec.rb

@@ -115,6 +115,15 @@ describe Gitlab::ReferenceExtractor do
     end
   end
 
+  it 'does not include anchors from table of contents in issue references' do
+    issue1 = create(:issue, project: project)
+    issue2 = create(:issue, project: project)
+
+    subject.analyze("not real issue <h4>#{issue1.iid}</h4>, real issue #{issue2.to_reference}")
+
+    expect(subject.issues).to match_array([issue2])
+  end
+
   it 'accesses valid issue objects' do
     @i0 = create(:issue, project: project)
     @i1 = create(:issue, project: project)