Assert what the escaper must guarantee, not one spelling of one tag

CodeQL opened a high-severity js/bad-tag-filter alert on the feed escaping
test: `assert.doesNotMatch(xml, /<script>/u)` does not match `<SCRIPT>`.

There is no vulnerability behind it. escapeXml works character by character
— every `<` becomes `&lt;` whatever follows it — so an upper case tag is
escaped exactly like a lower case one. What CodeQL found was the test, and
it was right about that: an assertion that hunts for one spelling of one tag
name would have stayed green while a real tag-name filter let `<SCRIPT>`
through.

So the assertion now states the invariant the escaper actually owes us:
nothing an agent writes comes back out of a description as markup. Not one
tag name — not a single angle bracket. Because it never looks at a tag name,
the case question disappears rather than being answered.

Measured rather than assumed. Against output from a filter with exactly that
flaw — `<script>` escaped, `<SCRIPT>` passed through verbatim — the old
assertion passes and the new one fails. Attempting the same proof by
weakening escapeXml itself was refused by the sandbox, correctly; the
comparison was made against a constructed string instead, and rss.ts was
never left broken in the tree.

Two smaller things the strict equality caught while writing it: the channel
carries its own `<description>` ahead of every item, so the first match in
the document is the wrong one, and the seed now carries both cases so the
expected string has to spell out what escaping both of them produces.

Local proof: 228 D1 tests pass, npm run check reports 0 errors.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sametbasbug

scripts/orbit-public-page-tests.ts

@@ -296,16 +296,42 @@ describe('Orbit dynamic public pages', () => {
     assert.match(xml, new RegExp(`<link>${link}</link><guid isPermaLink="true">${link}</guid>`, 'u'));
   });
 
+  /* Bu test `assert.doesNotMatch(xml, /<script>/u)` ile yazılmıştı ve CodeQL
+   * onu bir HTML filtresi sanıp yüksek seviye uyarı açtı (js/bad-tag-filter).
+   * Bulduğu şey doğruydu: küçük harfli etiketi arayan bir kalıp `<SCRIPT>`
+   * geçse sessizce yeşil kalırdı. Zafiyet değildi — escapeXml karakter bazlı
+   * çalışır, `<` her hâlde `&lt;` olur — zayıf olan testin kendisiydi.
+   *
+   * Artık iddia edilen şey gerçekten önemli olan değişmez: ajanın yazdığı
+   * hiçbir şey description'dan markup olarak geri dönmez. Tek bir etiket adı
+   * değil, tek bir açı parantezi bile. Etiket adına bakmadığı için hangi
+   * harfle yazıldığının da önemi kalmıyor. */
   test('escapes XML metacharacters in feed summaries', async () => {
+    const summary = 'Ajan & <script>alert("x")</script> <SCRIPT>alert("y")</SCRIPT> özeti';
     const response = await serveDynamicPublicPage(
       new Request('https://orbit.example/feed.xml'),
       assets,
-      new FakePublicRepository([record({ summary: 'Ajan & <script>alert("x")</script> özeti' })]),
+      new FakePublicRepository([record({ summary })]),
     );
     assert.ok(response);
     const xml = await response.text();
-    assert.doesNotMatch(xml, /<script>/u);
-    assert.match(xml, /Ajan &amp; &lt;script&gt;alert\(&quot;x&quot;\)&lt;\/script&gt; özeti/u);
+
+    /* Kanalın kendi description'ı item'lardan önce geliyor; sabit bir metin
+     * olduğu için ilk eşleşme testi sessizce yanlış yerden geçirir. */
+    const item = xml.indexOf('<item>');
+    assert.ok(item >= 0, 'feed carries an item');
+    const opening = '<description>';
+    const start = xml.indexOf(opening, item);
+    const end = xml.indexOf('</description>', start);
+    assert.ok(start >= 0 && end > start, 'feed item carries a description');
+    const description = xml.slice(start + opening.length, end);
+
+    assert.doesNotMatch(description, /[<>]/u);
+    assert.equal(
+      description,
+      'Ajan &amp; &lt;script&gt;alert(&quot;x&quot;)&lt;/script&gt;'
+      + ' &lt;SCRIPT&gt;alert(&quot;y&quot;)&lt;/SCRIPT&gt; özeti',
+    );
   });
 
   test('answers HEAD for the feed without a body', async () => {