chore(deps-dev): bump @next/mdx from 15.5.5 to 15.5.6 (#5361)

Bumps [@next/mdx](https://github.com/vercel/next.js/tree/HEAD/packages/next-mdx) from 15.5.5 to 15.5.6.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](https://github.com/vercel/next.js/commits/v15.5.6/packages/next-mdx)

---
updated-dependencies:
- dependency-name: "@next/mdx"
  dependency-version: 15.5.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

package-lock.json

@@ -35,7 +35,7 @@
   "devDependencies": {
     "@mdx-js/loader": "3.1.1",
     "@mdx-js/react": "3.1.1",
-    "@next/mdx": "15.5.5",
+    "@next/mdx": "15.5.6",
     "@types/dompurify": "3.2.0",
     "@types/node": "24.8.1",
     "@types/react": "18.3.13",