fix: externalize linked dependencies when loading explicit env

[svelte-triage-bot]

Fixes #16987.

The isolated Vite server used to evaluate src/env.ts externalized installed
dependencies but inlined linked workspace dependencies. This caused linked
packages to bypass native Node execution and prevented Node loader hooks from
resolving their imports.

Configure the isolated server with ssr.external: true so linked and installed
dependencies use consistent native Node semantics.

A regression test creates a linked package with an import resolved by a Node
loader hook and verifies that explicit environment loading succeeds.

Verification:

• Focused regression test
• Full @sveltejs/kit unit suite
• options-2 production build
• pnpm check
• pnpm lint
• Original issue reproduction

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 9975cd4:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/9975cd45fa3d88fca55ac3aaf6ba92e6096f8627

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16988

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: 9975cd4

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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