Match a paid Substack post's "Read more" link tolerantly (review item 8)

A trailing slash, tracking query, fragment, other scheme or host case on
either link no longer lets a paid post through the paid filter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/paywall_service_test.dart

@@ -56,6 +56,31 @@ void main() {
         reason: 'a free post ending with the subscribe widget');
   });
 
+  test('matches the "Read more" link of a paid Substack post to the post despite URL noise',
+      () async {
+    final paywall = PaywallService(await _storage(), client: MockClient((_) async => http.Response('', 500)));
+    Article cutPost({required String link, required String readMore}) => Article(
+          title: 'A paid post',
+          link: link,
+          source: _source(),
+          contentHtml: '<p>The first paragraph.</p><p><a href="$readMore">Read more</a></p>',
+        );
+    const post = 'https://example.substack.com/p/a-paid-post';
+
+    expect(paywall.isPaid(cutPost(link: post, readMore: '$post/')), isTrue,
+        reason: 'trailing slash on the "Read more" link');
+    expect(paywall.isPaid(cutPost(link: '$post/', readMore: post)), isTrue,
+        reason: 'trailing slash on the article link');
+    expect(paywall.isPaid(cutPost(link: post, readMore: '$post?utm_source=substack&utm_medium=email')),
+        isTrue, reason: 'tracking query on the "Read more" link');
+    expect(paywall.isPaid(cutPost(link: '$post?utm_campaign=post#footnote-1', readMore: post)), isTrue,
+        reason: 'query and fragment on the article link');
+    expect(paywall.isPaid(cutPost(link: post, readMore: 'http://Example.Substack.com/p/a-paid-post')),
+        isTrue, reason: 'other scheme and host case');
+    expect(paywall.isPaid(cutPost(link: post, readMore: 'https://example.substack.com/p/another-post')),
+        isFalse, reason: 'a "Read more" link to a different post');
+  });
+
   test('checks pages only for flagged sources and remembers results', () async {
     final storage = await _storage();
     final requested = <String>[];