fix: prevent state runes from being called with spread

[paoloricciuti]

This will make #15579 safer...is technically a breaking but we are probably safe from this.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15585-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15585

[changeset-bot]

🦋 Changeset detected

Latest commit: ae530e1

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

This PR includes changesets to release 1 package

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svelte	Patch

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