fix: set server context after async transformError (#17799)

This will get the tests in https://github.com/sveltejs/kit/pull/15308
green, right now they fail because page state cannot be found because
context not available

.changeset/gentle-emus-walk.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: set server context after async transformError

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

@@ -293,6 +293,7 @@ export class Renderer {
 				}
 
 				child.promise = /** @type {Promise<unknown>} */ (result).then((transformed) => {
+					set_ssr_context(parent_context);
 					child.#out.push(`<!--${HYDRATION_START_FAILED}${JSON.stringify(transformed)}-->`);
 					failed_snippet(child, transformed, noop);
 					child.#out.push(BLOCK_CLOSE);

packages/svelte/tests/runtime-runes/samples/async-error-boundary-3/_config.js

@@ -0,0 +1,15 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['hydrate', 'async-server', 'client'],
+	ssrHtml: '<p>caught: error (hello)</p>',
+	transformError: () => {
+		return Promise.resolve('error');
+	},
+
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>caught: error (hello)</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-error-boundary-3/child.svelte

@@ -0,0 +1,14 @@
+<script>
+	import { get } from "./main.svelte";
+
+    let { error } = $props();
+    const context = get()
+</script>
+
+{#if error}
+    <p>caught: {await error} ({context})</p>
+{:else}
+	{(() => {
+        throw 'catch me';
+    })()}
+{/if}

packages/svelte/tests/runtime-runes/samples/async-error-boundary-3/main.svelte

@@ -0,0 +1,19 @@
+<script module>
+	import { createContext } from "svelte";
+	import Child from "./child.svelte";
+
+	const [ get, set ] = createContext();
+	export {get};
+</script>
+
+<script>
+	set('hello');
+</script>
+
+<svelte:boundary>
+	{#snippet failed(error)}
+		<Child {error} />
+	{/snippet}
+
+	<Child />
+</svelte:boundary>