fix: correctly handle bindings on the server (#18009)

`item.subsume(item)` did nothing, honestly kinda weird how this only
turned up now

Fixes #17981

.changeset/eight-trees-occur.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly handle bindings on the server

packages/svelte/src/internal/server/renderer.js

@@ -468,10 +468,14 @@ export class Renderer {
 		}
 
 		this.local = other.local;
-		this.#out = other.#out.map((item) => {
+		this.#out = other.#out.map((item, i) => {
-			if (item instanceof Renderer) {
+			const current = this.#out[i];
-				item.subsume(item);
+
+			if (current instanceof Renderer && item instanceof Renderer) {
+				current.subsume(item);
+				return current;
 			}
+
 			return item;
 		});
 		this.promise = other.promise;

packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Async.svelte

@@ -0,0 +1,7 @@
+<script>
+	let data = $derived(await Promise.resolve('test'));
+</script>
+
+<div data-resolved={data ? 'true' : 'false'}>
+	{data}
+</div>

packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Binding.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Bound from './Bound.svelte';
+
+	let open;
+</script>
+
+<Bound bind:open />

packages/svelte/tests/runtime-runes/samples/async-hydration-binding/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+// Tests that renderer.subsume (which is used when bindings are present) works correctly
+export default test({
+	mode: ['hydrate'],
+	html: '<div data-resolved="true">test</div>',
+	async test({ assert, warnings }) {
+		assert.deepEqual(warnings, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-hydration-binding/main.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+	import Async from './Async.svelte';
+	import Binding from './Binding.svelte';
+</script>
+
+<Async />
+<Binding />