fix: export `wait` function from internal client index (#17530)

PR #17461 added the `wait` function to `reactivity/async.js` and modified the compiler to generate `$.wait()` calls, but forgot to export it from `index.js`.

This causes a runtime error when using `await` inside `$derived()` with `experimental.async: true`:

```
TypeError: $.wait is not a function
```

Fixes #17529

.changeset/fix-wait-export.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: export `wait` function from internal client index

packages/svelte/src/internal/client/index.js

@@ -103,7 +103,8 @@ export {
 	run,
 	save,
 	track_reactivity_loss,
-	run_after_blockers
+	run_after_blockers,
+	wait
 } from './reactivity/async.js';
 export { eager, flushSync as flush } from './reactivity/batch.js';
 export {

packages/svelte/tests/runtime-runes/samples/async-const-wait/_config.js

@@ -0,0 +1,18 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+
+	props: {
+		a_promise: Promise.resolve(10),
+		b_promise: Promise.resolve(20)
+	},
+
+	async test({ assert, target }) {
+		await tick();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, `<p>30</p>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-const-wait/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	let { a_promise, b_promise } = $props();
+</script>
+
+<svelte:boundary>
+	{@const a = await a_promise}
+	{#if true}
+		<svelte:boundary>
+			{@const b = await b_promise}
+			{#if true}
+				{@const sum = a + b}
+				<p>{sum}</p>
+			{/if}
+		</svelte:boundary>
+	{/if}
+</svelte:boundary>