fix: Use STATE_SYMBOL to de-proxy component in unmount (#16656)

[pranav200408]

loses #16656

Description

This PR addresses a bug where calling the unmount method on a component instance created with $state would fail due to the instance being a proxy.

Fix

The unmount method now correctly retrieves the component's raw instance using component[STATE_SYMBOL] from the proxy, allowing it to be properly found in the mounted_components map.

Tests

• Ran pnpm test and all tests passed.
• Ran pnpm lint and all lint checks passed.

Additional Information

This change implements the solution suggested by maintainers in the issue thread, ensuring a correct fix that aligns with Svelte's internal design.

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Latest commit: 594d35e

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16714-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16714

[dummdidumm]

Thanks for starting the work on this, but

• this needs a test (add one in runtime-runes) testing the fixed behavior
• this needs to make sure it does not regress in other places (this will now fail if you didn't proxify the component instance with state)
• this needs a change set (run pnpm changeset at the root and follow the prompts)

[Rich-Harris]

closing in favour of #16747, there's just no reason to use $state here in the first place and thus no reason to accommodate it