fix: point blog "Edit on GitHub" link at the source file

src/pages/[lang]/blog/[slug].astro

@@ -61,7 +61,7 @@ const relatedPosts = [...tagged, ...untagged].slice(0, 3);
         as="a"
         variant="secondary"
         size="sm"
-        href={`https://github.com/expressjs/expressjs.com/edit/main/src/content/blog/${Astro.params.slug}`}
+        href={`https://github.com/expressjs/expressjs.com/edit/main/${post.filePath}`}
       >
         <Icon name="github" aria-hidden="true" />
         {t('page.editOnGitHub')}

tests/e2e/blog.spec.ts

@@ -0,0 +1,32 @@
+import { test, expect } from '@playwright/test';
+import enStrings from '../../src/i18n/ui/en.json' with { type: 'json' };
+
+const EDIT_URL_BASE = 'https://github.com/expressjs/expressjs.com/edit/main/src/content/blog';
+
+/**
+ * Blog Post E2E Tests
+ *
+ * The post URL is a lowercased slug without an extension, so the edit link has to
+ * point at the source file itself. Cover a mixed-case `.md` and a mixed-case `.mdx`.
+ */
+const posts = [
+  {
+    url: '/en/blog/2024-10-01-herodevs-partnership-announcement',
+    file: '2024-10-01-HeroDevs-partnership-announcement.md',
+  },
+  {
+    url: '/en/blog/2026-10-28-cve-2026-111181',
+    file: '2026-10-28-CVE-2026-111181.mdx',
+  },
+];
+
+test.describe('Blog Post', () => {
+  for (const { url, file } of posts) {
+    test(`should link to the source file of ${file} for editing`, async ({ page }) => {
+      await page.goto(url);
+
+      const editLink = page.getByRole('link', { name: enStrings.page.editOnGitHub });
+      await expect(editLink).toHaveAttribute('href', `${EDIT_URL_BASE}/${file}`);
+    });
+  }
+});