fix: ensure batch exists when resetting a failed boundary

fixes #16681

.changeset/ninety-ravens-join.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure batch exists when resetting a failed boundary

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

@@ -297,6 +297,9 @@ export class Boundary {
 				e.svelte_boundary_reset_onerror();
 			}
 
+			// If the failure happened while flushing effects, current_batch can be null
+			Batch.ensure();
+
 			this.#pending_count = 0;
 
 			if (this.#failed_effect !== null) {

packages/svelte/tests/runtime-runes/samples/async-boundary-reset/Test.svelte

@@ -0,0 +1,15 @@
+<script>
+	async function c(a) {
+			await Promise.resolve()
+			if (a) {
+				throw new Error('error');
+			} else {
+				return 'ok';
+			}
+	}
+
+	let a = $state();
+	let b = $derived(await c(a));
+</script>
+
+<button onclick={() => a = 1}>{b}</button>

packages/svelte/tests/runtime-runes/samples/async-boundary-reset/_config.js

@@ -0,0 +1,19 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		let [btn] = target.querySelectorAll('button');
+		btn.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>reset</button>');
+
+		[btn] = target.querySelectorAll('button');
+		btn.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>ok</button>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-boundary-reset/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	import Test from './Test.svelte';
+</script>
+
+<svelte:boundary>
+	<Test />
+
+	{#snippet pending()}pending{/snippet}
+	{#snippet failed(_, reset)}
+		<button onclick={reset}>reset</button>
+	{/snippet}
+</svelte:boundary>