fix(make-pdf): stop URLs swallowing smartypants placeholders

A bare autolinked URL (<a href="X">X</a>) has zero whitespace between
the URL text and its own closing tag. TAG_RE carves that </a> into a
NUL-delimited SMARTPANTS_PRESERVED placeholder BEFORE the URL pass
runs, and URL_RE's \S+ swallowed the adjacent placeholder into the URL
match. The restore pass is single-shot, so the inner placeholder never
restored: raw "SMARTPANTS_PRESERVED_N" text leaked into the rendered
link, the </a> vanished, and link-blue styling bled into the rest of
the document (#2084). Excluding the NUL sentinel (\u0000) from the URL
character class stops the match from crossing into an already-carved
zone.

Contributed by @marshaung (PR #2280; PR #2339 by @BrendaB24 covered the
same smartypants defect).

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

make-pdf/test/render.test.ts

@@ -56,6 +56,34 @@ describe("smartypants", () => {
     expect(out).toContain("\u201cdetails\u201d");
   });
 
+  // A URL flush against a following tag (no whitespace between) used to let
+  // URL_RE's \S+ swallow the placeholder standing in for that tag. The
+  // placeholder never restored, so the tag vanished and its styling bled
+  // into the rest of the document. The test above misses this because its
+  // URL is followed by a space.
+  test("does not leak placeholders for a bare autolinked URL", () => {
+    const input = `<p>see <a href="https://ex.com">https://ex.com</a> ok</p>`;
+    const out = smartypants(input);
+    expect(out).not.toContain("SMARTPANTS_PRESERVED");
+    expect(out).toContain("</a>");
+    expect(out).toBe(input);
+  });
+
+  test("does not leak placeholders for a bold URL", () => {
+    const input = `<p>see <strong><a href="https://ex.com">https://ex.com</a></strong> ok</p>`;
+    const out = smartypants(input);
+    expect(out).not.toContain("SMARTPANTS_PRESERVED");
+    expect(out).toContain("</a></strong>");
+    expect(out).toBe(input);
+  });
+
+  test("does not leak placeholders when a URL is followed by punctuation", () => {
+    const input = `<p>see <a href="https://ex.com">https://ex.com</a>, ok</p>`;
+    const out = smartypants(input);
+    expect(out).not.toContain("SMARTPANTS_PRESERVED");
+    expect(out).toContain("</a>");
+  });
+
   test("does NOT touch HTML attribute values", () => {
     const out = smartypants(`<a href="it's-a-test.html">link</a>`);
     expect(out).toContain(`href="it's-a-test.html"`);