Bump next from 15.5.7 to 15.5.9

Bumps [next](https://github.com/vercel/next.js) from 15.5.7 to 15.5.9.
- [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/compare/v15.5.7...v15.5.9)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.9
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -6,7 +6,7 @@
     "postbuild": "pagefind --site .next/server/app --output-path public/_pagefind"
   },
   "dependencies": {
-    "next": "^15.5.7",
+    "next": "^15.5.9",
     "nextra": "^4.4.0",
     "nextra-theme-docs": "^4.4.0",
     "react": "^19.1.1",