fix: wrap `Promise.all` in `save` during SSR (#18178)

Closes #18168

Not sure if there's a deeper issue in play because the error it's only
really there if you also add a title to the component. I think the issue
is that with multiple arguments the top level `Promise.all` is not
wrapped in `save` and that probably causes a race condition with `title`
that sets the context back to `null` in a `finally`.

One issue is that now the generated code looks like this

```js
const [$$0, $$1] = (await $.save(Promise.all([
	(async () => (await $.save(user()))().name)(),
	(async () => (await $.save(user()))().image)()
])))();
```
which seems a bit redundant, but I'm not sure if we can get rid of the
inner `save` since they are indeed awaiting something.

.changeset/tough-knives-smell.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: wrap `Promise.all` in `save` during SSR

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js

@@ -12,7 +12,7 @@ import {
 import * as b from '#compiler/builders';
 import { sanitize_template_string } from '../../../../../utils/sanitize_template_string.js';
 import { regex_whitespaces_strict } from '../../../../patterns.js';
-import { has_await_expression } from '../../../../../utils/ast.js';
+import { has_await_expression, save } from '../../../../../utils/ast.js';
 import { ExpressionMetadata } from '../../../../nodes.js';
 
 /** Opens an if/each block, so that we can remove nodes in the case of a mismatch */
@@ -360,7 +360,7 @@ export class PromiseOptimiser {
 
 		return b.const(
 			b.array_pattern(this.expressions.map((_, i) => b.id(`$$${i}`))),
-			b.await(b.call('Promise.all', promises))
+			save(b.call('Promise.all', promises))
 		);
 	}
 

packages/svelte/tests/server-side-rendering/samples/async-multiple-attrs/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['async'],
+	compileOptions: {
+		dev: true
+	}
+});

packages/svelte/tests/server-side-rendering/samples/async-multiple-attrs/_expected.html

@@ -0,0 +1 @@
+<!--[--><img alt="test" src=""/><!--]-->

packages/svelte/tests/server-side-rendering/samples/async-multiple-attrs/_expected_head.html

@@ -0,0 +1 @@
+<!--1410iyz--><!----><title>Async multiple attributes</title>

packages/svelte/tests/server-side-rendering/samples/async-multiple-attrs/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	const user = $derived(Promise.resolve({ 
+		name: 'test',
+		image: '',
+	}))
+</script>
+
+<svelte:head>
+<title>Async multiple attributes</title>
+</svelte:head>
+
+<img
+	alt={(await user).name}
+	src={(await user).image}
+/>