fix(docs): redirect any-case error-code URLs to canonical uppercase

Error reference pages are served at /errors/DFxxxx from the content filenames, so lowercase deep links like /errors/df8111 404. Add a server middleware that 308-redirects any-case DFxxxx error-code paths to their canonical uppercase URL, mirroring the existing trailing-slash middleware.

Author: antfubot

docs/server/middleware/error-code-case.ts

@@ -0,0 +1,14 @@
+/**
+ * Error reference pages live at `/errors/DFxxxx` (uppercase, derived from the
+ * content filenames). Accept any-case deep links like `/errors/df8111` and
+ * redirect them to the canonical uppercase URL so a single URL is indexed.
+ */
+export default defineEventHandler((event) => {
+  const match = /^\/errors\/(df\d{4})(?=\/|$|\?)/i.exec(event.path)
+  if (!match)
+    return
+  const canonical = match[1].toUpperCase()
+  if (match[1] === canonical)
+    return // already canonical, let it through
+  return sendRedirect(event, event.path.replace(match[1], canonical), 308)
+})