fix: throw on invalid {@tag}s

[Ocean-OS]

Closes #17252

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[changeset-bot]

🦋 Changeset detected

Latest commit: 23a32c5

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

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svelte	Patch

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[Rich-Harris]

ah whoops, approved before seeing the typechecking error

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17256-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17256