ci: run the built Worker and request every page

[anurag629]

What and why

v0.7.0 passed CI and the Workers build, then threw on every request in production (Next 16.4 with OpenNext 1.20.10, fixed in #184). Nothing in CI ever ran the built Worker, so a green next build was all we knew.

The verify job now builds with opennextjs-cloudflare build (it runs npm run build itself, so the plain build step is gone), starts the result with wrangler dev and requests:

• every page in /sitemap.xml, so a new tool is covered without touching CI
• the og:image each page links to (the tool images have a hash in their path, so they're read from the page rather than guessed)
• /sitemap.xml, /robots.txt, and a missing page, which has to be 404

Any other status, or a [wrangler:error] line in the log, fails the step and prints the wrangler output. No secrets are needed, so it works on fork PRs too. It's in the same job, so verify stays the only check to look at.

CONTRIBUTING mentions the new step and how to reproduce a failure with npm run preview.

How I tested this

Ran the exact step script locally on Node 22 after npm ci and opennextjs-cloudflare build:

• On develop (315e056): all 6 pages and their 6 images return 200, /no-such-page returns 404, step passes.
• On the v0.7.0 commit (7ad22cb, next 16.4.0 with @opennextjs/cloudflare 1.20.10): /sitemap.xml, /robots.txt and the missing page all return 500 with Unexpected loadManifest(/.next/server/preview-props.json) call!, which is the production error, and the step fails.

On GitHub's runner (this PR's CI) the step made the same 15 requests and passed.

Type of change

• Tests

Notes for reviewers

verify took 71 s on this PR. The last four runs on develop took 40 to 60 s, so it adds about 15 to 30 s. That's low because the separate next build step is gone.