fix: ownership getting tricked by proxies

[paoloricciuti]

Svelte 5 rewrite

Closes #13376

This fixes the problem but i fear we might have this problem elsewhere and this can still be somewhat tricked unless we specify a lot of constraint. The problem when someone returns some non null value from a proxy regardless of the key eg

<script>
    import { setContext, getContext } from "svelte";

    setContext("", new Proxy({}, {
        get(){
            return {};
        }
    }));

    getContext("");
</script>

inside the ownership validation function we assume that if it has metadata it's the svelte metadata object (honestly a reasonable assumption considering it's a Symbol). But in this case the object will always be this empty object. So when trying to access owners.add on it or [ADD_OWNER] we end up with a runtime error.

By adding owners in object check it's fixed but that will fail if the proxy looks like this

<script>
    import { setContext, getContext } from "svelte";

    setContext("", new Proxy({}, {
        get(){
            return {};
        },
        has(){
             return true;
        }
    }));

    getContext("");
</script>

So i'm not really sure if we should go mad with checks before invoking anything or find a better general solution.

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