fix: omit ISR data endpoints for server-only routes

[shixi-li]

closes #12690

Server-only ISR routes do not have a page data endpoint, but the Vercel adapter currently emits a data function, prerender configuration, and routing entry for them.

This change only creates those data-specific ISR artifacts when a route has page methods. The server route main function, prerender configuration, and routing entry remain unchanged.

The build regression adds ISR to the existing API fixture and verifies that its main artifacts are present while its data artifacts are absent. It also verifies that a regular ISR page still keeps its data function and route.

---

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Tests

• pnpm -C packages/adapter-vercel test
• pnpm lint
• pnpm check
• pnpm -F @sveltejs/kit test:unit

Changesets

• Added a patch changeset for @sveltejs/adapter-vercel.

Edits

• Allow edits from maintainers is enabled.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 0e50024:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/0e50024c99994ce7855fb1b79c5b66530fe3bad9

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16731

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0e50024

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-vercel	Patch

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[shixi-li]

Rebased this PR onto the latest version-3 (bc07967c7) and resolved the one package-script conflict by preserving upstreams removal of the prepare script while retaining this PRs post-build assertion. The functional diff remains scoped to the same five files; git diff --check passes.

I attempted both the adapters complete test command and its focused unit/build halves locally. In this workspace each hung without output until the command timeout/interrupt, so I did not claim a local pass for the rebased head. The previous heads GitHub CI was green; the newly pushed head is 0e50024c9 and will get a fresh CI signal.