fix(next-mdx): pin .mdx server pages to the rsc bundleLayer - metadata exports build under webpack

[icyJoseph]

An App Router .mdx page that does export const metadata = {…} fails to build with webpack:

You are attempting to export "metadata" from a component marked with "use client"

@next/mdx registers its .mdx rule with the layer-agnostic options.defaultLoaders.babel, so the SWC pass runs with bundleLayer: undefined. That makes isReactServerLayer false, and as of 16.2 the RSC metadata guard rejects any non-react-server module that exports metadata, even without an actual "use client" directive. A .tsx page in the same dir is unaffected because Next's core layered rules only match /\.(tsx|ts|js|cjs|mjs|jsx)$/, so .mdx never gets the rsc loader and falls through to the layer-less one.

One possible fix is to pin the MDX SWC pass to the rsc bundle layer (App Router .mdx pages are Server Components), during the server compile step.

Also added app/metadata/page.mdx + an assertion in test/e2e/app-dir/mdx/mdx.test.ts (runs for both mdx-rs and non-rs) verifying the exported metadata renders.

Fixes #91735

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Failing test suites

Commit: 6575da2 | About building and testing Next.js

pnpm test-dev test/development/app-dir/hmr-intercept-routes/hmr-intercept-routes.test.ts (job)

• hmr-intercept-routes > should update intercept routes via HMR (DD)

Expand output

● hmr-intercept-routes › should update intercept routes via HMR

page.waitForSelector: Timeout 10000ms exceeded.
Call log:
  - waiting for locator('#default-intercept') to be visible

  569 |
  570 |     return this.startChain(async () => {
> 571 |       const el = await page.waitForSelector(selector, {
      |                             ^
  572 |         timeout,
  573 |         state,
  574 |       })

  at waitForSelector (lib/browsers/playwright.ts:571:29)
  at Playwright._chain (lib/browsers/playwright.ts:701:23)
  at Playwright._chain [as startChain] (lib/browsers/playwright.ts:682:17)
  at Playwright.startChain [as waitForElementByCss] (lib/browsers/playwright.ts:570:17)
  at Object.waitForElementByCss (development/app-dir/hmr-intercept-routes/hmr-intercept-routes.test.ts:38:21)

[vercel]

The @next/mdx plugin forces bundleLayer: 'rsc' on ALL server-side .mdx modules (gated only by options.isServer), so Pages Router .mdx pages and App Router .mdx files imported by client components are wrongly run through the React Server Components server-layer SWC transform.