fix: never let a '<' survive title flattening

CodeQL flagged the tag strip in plainTitle: requiring a closing '>' means
a title whose markup never closes keeps its '<script' fragment. The '>'
is optional now, so every '<' starts a strip and none reaches the results
page, and the output is trimmed. Both call sites already HTML-escape the
result; this hardens the flattening itself. Covered by a new test.

src/sphinx.js

@@ -151,8 +151,12 @@ export function searchIndex(index, query) {
 
 // Titles in the index arrive as the HTML of the page's <h1>, markup and all
 // (docs.python.org wraps module names in <code> spans), so they are flattened
-// to text before they are placed on the results page.
-const plainTitle = (t) => String(t).replace(/<[^>]*>/g, '');
+// to text before they are placed on the results page. The '>' is optional so
+// a tag the title never closes is dropped too, not left standing as '<script'
+// for some later layer to trip on: a title is index data from the network,
+// and no '<' survives this. A literal less-than in a real title arrives as
+// '&lt;', so nothing legitimate is lost.
+const plainTitle = (t) => String(t).replace(/<[^>]*>?/g, '').trim();
 
 const escapeHTML = (s) => String(s)
   .replaceAll('&', '&amp;').replaceAll('<', '&lt;')

tests/sphinx.test.js

@@ -43,6 +43,15 @@ test('titles are flattened to text before they reach the results page', () => {
   assert.equal(found.docs[0].title, 'json - JSON encoder and decoder');
 });
 
+test('a tag the title never closes is stripped, not left standing', () => {
+  // 'json encoder <script src=' has no closing '>', so a strip that requires
+  // one would hand '<script' onward. No '<' may survive the flattening.
+  const nested = { ...INDEX, titles: ['json encoder <script src=', ...INDEX.titles.slice(1)] };
+  const found = searchIndex(nested, 'json');
+  assert.equal(found.docs[0].title, 'json encoder');
+  assert.doesNotMatch(resultsToHTML(BASE, 'json', found, nested), /<script/);
+});
+
 test('every word must match, and when none can, any-word results say so', () => {
   // 'json' hits doc 0, 'socket' hits doc 2, nothing hits both.
   const found = searchIndex(INDEX, 'json socket');