fix: ignore mutation validation for props that are not proxies in more cases

[dummdidumm]

This fixes an off-by-one error - we did not bail if the top level of the prop wasn't a state already. Fixes #15727

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🦋 Changeset detected

Latest commit: 9ccf8a5

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

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svelte	Patch

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Preview: https://svelte-dev-git-preview-svelte-15759-svelte.vercel.app/

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