fix: check boundary pending attribute at runtime on server

[Ocean-OS]

See sveltejs/kit#14447 (comment)

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[changeset-bot]

🦋 Changeset detected

Latest commit: 2407d96

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16855

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16855-svelte.vercel.app/

[Rich-Harris]

love it!