fix(next-mdx): preserve MDX metadata in webpack

[mturac]

What

Teach @next/mdx to pass app-router MDX files through next-swc-loader with the React Server Components bundle layer when webpack is used.

This keeps MDX pages that export metadata classified as Server Components instead of being rejected as Client Components.

Why

The documented app-router MDX pattern supports export const metadata, but under webpack the custom MDX rule reused the default SWC loader without the RSC bundle layer. That made app/page.mdx fail during next build --webpack with:

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

How

• Clone the default SWC loader used by @next/mdx webpack rules.
• Add bundleLayer: "rsc" only when that loader is next-swc-loader.
• Extend the existing MDX e2e fixture with a metadata export and assert the rendered <title>.

Fixes #91735

Testing

• pnpm build-all
• HEADLESS=true pnpm test-start-webpack test/e2e/app-dir/mdx/mdx.test.ts -t "should work in initial html"
• HEADLESS=true pnpm test-start-webpack test/e2e/app-dir/mdx/mdx.test.ts
• HEADLESS=true pnpm test-start-turbo test/e2e/app-dir/mdx/mdx.test.ts
• node --check packages/next-mdx/index.js
• pnpm prettier --check packages/next-mdx/index.js test/e2e/app-dir/mdx/mdx.test.ts test/e2e/app-dir/mdx/app/page.mdx
• pnpm --filter @next/mdx pack-for-isolated-tests
• git diff --check