chore: gracefully handle 404s from pages router's dynamic pages + tests (#18618)

* restore pages/_error

* set custom header in pages/_error

* handle it in middleware

* add test

* remove logs

* better test description

apps/web/middleware.ts

@@ -22,6 +22,11 @@ const safeGet = async <T = any>(key: string): Promise<T | undefined> => {
 const middleware = async (req: NextRequest): Promise<NextResponse<unknown>> => {
   const url = req.nextUrl;
   const requestHeaders = new Headers(req.headers);
+  const response = NextResponse.next();
+
+  if (response.headers.get("x-pages-router-error") === "true") {
+    return NextResponse.rewrite(new URL("/_not-found", req.url));
+  }
 
   requestHeaders.set("x-url", req.url);