FIX: Always display error page in Next.js app locally

[amoore108]

Summary

Small fix to always display an error page if the route being viewed locally is incorrect.

Previously just a blank white page would be displayed for a route that didn't match.

This is just a local development aid, as upstream systems define "valid" routes for the application and return a static 404 page in instances that routes aren't matched.

Testing

1. Visit:
   http://localhost:7081/foobar
   http://localhost:7081/foobar.amp
   http://localhost:7081/igbo/foobar
   http://localhost:7081/igbo/foobar.amp
2. Confirm all the links return a 404 error page, with the non-Igbo pages defaulting to English

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[Copilot]

Pull request overview

This PR improves the local developer experience in the ws-nextjs-app Next.js application by ensuring invalid local routes consistently render the standard 404 error page (instead of a blank screen), even when the “service” segment in the URL is not a recognised Simorgh service.

Changes:

• Validates the service extracted from the URL against the canonical services list and falls back to news when invalid, so the error page can render.
• Replaces hard-coded 404 values with the shared NOT_FOUND status constant.