fix: settle discarded batch (#18290)

Without this the promise would never resolve

.changeset/neat-groups-grin.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: settle discarded batch

packages/svelte/src/internal/client/reactivity/batch.js

@@ -633,6 +633,7 @@ export class Batch {
 		this.#fork_commit_callbacks.clear();
 
 		this.#unlink();
+		this.#deferred?.resolve();
 	}
 
 	/**

packages/svelte/tests/runtime-runes/samples/async-settled-discard/_config.js

@@ -0,0 +1,37 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+		const [increment, pop] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+		increment.click();
+		await tick();
+		pop.click();
+		await tick();
+		assert.deepEqual(logs, ['settled 2', 'settled 2']);
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				2
+				<button>increment</button>
+				<button>pop</button>
+			`
+		);
+
+		pop.click();
+		await tick();
+		assert.deepEqual(logs, ['settled 2', 'settled 2']);
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				2
+				<button>increment</button>
+				<button>pop</button>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-settled-discard/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	import { settled } from "svelte";
+
+	let count = $state(0);
+	let queued = [];
+
+	function push(v) {
+		if (!v) return v;
+
+		return new Promise((resolve) => {
+			queued.push(() => resolve(v));
+		});
+	}
+</script>
+
+{await push(count)}
+<button onclick={async () => {
+	count++;
+	await settled();
+	console.log('settled ' + count);
+}}>increment</button>
+<button onclick={() => queued.pop()?.()}>pop</button>