fix: preserve original boundary errors when keyed each rows are removed during async updates (#17843)

Fixes a runtime edge case where keyed #each reconciliation can hit a
missing item during deferred async updates, causing an internal crash
and masking the original boundary error.

Fixes #17841

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

---------

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

.changeset/odd-badgers-camp.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: preserve original boundary errors when keyed each rows are removed during async updates

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -35,7 +35,7 @@ import {
 } from '../../reactivity/effects.js';
 import { source, mutable_source, internal_set } from '../../reactivity/sources.js';
 import { array_from, is_array } from '../../../shared/utils.js';
-import { BRANCH_EFFECT, COMMENT_NODE, EFFECT_OFFSCREEN, INERT } from '#client/constants';
+import { BRANCH_EFFECT, COMMENT_NODE, DESTROYED, EFFECT_OFFSCREEN, INERT } from '#client/constants';
 import { queue_micro_task } from '../task.js';
 import { get } from '../../runtime.js';
 import { DEV } from 'esm-env';
@@ -217,6 +217,10 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 	 * @param {Batch} batch
 	 */
 	function commit(batch) {
+		if ((state.effect.f & DESTROYED) !== 0) {
+			return;
+		}
+
 		state.pending.delete(batch);
 
 		state.fallback = fallback;

packages/svelte/tests/runtime-runes/samples/async-each-const-await-error-boundary/_config.js

@@ -0,0 +1,28 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	// this test doesn't fail without the associated fix — the error gets
+	// swallowed somewhere. but keeping it around for illustration
+	skip: true,
+
+	mode: ['client'],
+
+	async test({ assert, target, errors, logs }) {
+		const button = target.querySelector('button');
+
+		button?.click();
+		await tick();
+		await tick();
+		assert.deepEqual(logs, ['Simulated TypeError']);
+		assert.deepEqual(errors, []);
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>Trigger</button>
+				<p>Error Caught: Simulated TypeError</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-each-const-await-error-boundary/main.svelte

@@ -0,0 +1,29 @@
+<script>
+	let index = $state(0);
+
+	async function fn(id) {
+		if (id === 2) throw new Error('Simulated TypeError');
+		return id;
+	}
+
+	function onerror(error) {
+		console.log(error.message);
+	}
+</script>
+
+<button onclick={() => (index = 1)}>Trigger</button>
+
+<svelte:boundary {onerror}>
+	{#snippet pending()}
+		<p>Loading...</p>
+	{/snippet}
+
+	{#snippet failed(error)}
+		<p>Error Caught: {error.message}</p>
+	{/snippet}
+
+	{#each [[1], [2]][index] as id (id)}
+		{@const result = await fn(id)}
+		<p>{result}</p>
+	{/each}
+</svelte:boundary>