fix: each block losing reactivity when items removed while promise pending (#17151)

Co-authored-by: dylan <dylanbradshaw107@hotmail.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

Author: 3 people

.changeset/clean-toes-joke.md

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+---
+'svelte': patch
+---
+
+fix: each block losing reactivity when items removed while promise pending

packages/svelte/tests/runtime-runes/samples/async-each-derived/_config.js

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+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick(); // settle initial await
+
+		const checkBox = target.querySelector('input');
+
+		checkBox?.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<input type="checkbox"/>
+				<p>true</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-each-derived/main.svelte

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+<script>
+	let checked = $state(false);
+
+	const foo = $derived(await checked);
+</script>
+
+<input type="checkbox" bind:checked />
+
+{#each checked === foo && [1]}
+	<p>{checked}</p>
+{/each}