test: check the edge bundle for treeshaking in the build test

[Nic-Polumeyv]

The Netlify edge app's Playwright test reads the built edge function from disk, but since #17122 the platform test job runs against a deployment and has no build output. This moves that assertion into build.test.js, next to the other build output checks.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from e0b74d4:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/e0b74d4838d9f592f56f14fb41a61345cfaf5c8b

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17172

[changeset-bot]

⚠️ No Changeset found

Latest commit: e0b74d4

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-17172-svelte.vercel.app/