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/playwright/app-router-not-found.e2e.ts

@@ -0,0 +1,15 @@
+import { expectPageToBeNotFound } from "playwright/lib/testUtils";
+
+import { test } from "./lib/fixtures";
+
+test.describe("App Router - error handling", () => {
+  test("404s must render App Router's not-found page regardless of whether pages router or app router is used", async ({
+    page,
+  }) => {
+    await expectPageToBeNotFound({ page, url: "/123491234" });
+    await expectPageToBeNotFound({ page, url: "/team/123491234" });
+    await expectPageToBeNotFound({ page, url: "/org/123491234" });
+    await expectPageToBeNotFound({ page, url: "/insights/123491234" });
+    await expectPageToBeNotFound({ page, url: "/login/123491234" });
+  });
+});