test: add more guard-derived regression samples

Author: hmnd

packages/svelte/tests/runtime-runes/samples/guard-derived-nested-multi/_config.js

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+import { test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	mode: ['client'],
+	async test({ target }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+
+		const text = target.textContent?.trim() ?? '';
+		if (!text.endsWith('advance')) {
+			throw new Error(`unexpected text: ${text}`);
+		}
+	}
+});

packages/svelte/tests/runtime-runes/samples/guard-derived-nested-multi/main.svelte

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+<script>
+	import Wrapper from './wrapper.svelte';
+
+	let rows = $state([
+		{ nested: { depth: 1 } },
+		{ nested: { depth: 3 } }
+	]);
+	let ptr = $state(0);
+
+	let current = $derived(rows[ptr]);
+	let currentLength = $derived(current?.length);
+	let nested = $derived(current?.nested);
+
+	$effect(() => {
+		if (ptr === 1) {
+			rows = [];
+		}
+	});
+</script>
+
+<Wrapper>
+	{#if currentLength && nested?.depth > 2}
+		deep
+	{:else}
+		shallow
+	{/if}
+</Wrapper>
+
+<div>
+	{#if nested?.depth > 2}
+		deep
+	{:else}
+		shallow
+	{/if}
+</div>
+
+<button onclick={() => (ptr = 1)}>advance</button>

packages/svelte/tests/runtime-runes/samples/guard-derived-nested-multi/wrapper.svelte

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+<script>
+	let { children } = $props();
+</script>
+
+{@render children()}