fix: throw for unset `createContext` get on the server (#17580)

* fix: throw for unset `createContext` get on the server

* smol tweak

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/better-groups-swim.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: throw for unset `createContext` get on the server

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

@@ -17,7 +17,17 @@ export function set_ssr_context(v) {
  */
 export function createContext() {
 	const key = {};
-	return [() => getContext(key), (context) => setContext(key, context)];
+
+	return [
+		() => {
+			if (!hasContext(key)) {
+				e.missing_context();
+			}
+
+			return getContext(key);
+		},
+		(context) => setContext(key, context)
+	];
 }
 
 /**

packages/svelte/tests/server-side-rendering/samples/context-not-set-throws/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	error: 'missing_context'
+});

packages/svelte/tests/server-side-rendering/samples/context-not-set-throws/main.svelte

@@ -0,0 +1,9 @@
+<script module>
+	import { createContext } from 'svelte';
+
+	const [get] = createContext();
+</script>
+
+<script>
+	get();
+</script>

packages/svelte/tests/server-side-rendering/test.ts

@@ -12,6 +12,7 @@ import { assert_html_equal_with_options } from '../html_equal.js';
 import { suite_with_variants, type BaseTest } from '../suite.js';
 import type { CompileOptions } from '#compiler';
 import { seen } from '../../src/internal/server/dev.js';
+import type { SyncRenderOutput } from '#server';
 
 interface SSRTest extends BaseTest {
 	mode?: ('sync' | 'async')[];
@@ -76,6 +77,9 @@ const { test, run } = suite_with_variants<SSRTest, 'sync' | 'async', CompileOpti
 
 		let rendered;
 		let errored = false;
+		let body: SyncRenderOutput['body'];
+		let head: SyncRenderOutput['head'];
+		let hashes: SyncRenderOutput['hashes'];
 		try {
 			const render_result = render(Component, {
 				props: config.props || {},
@@ -83,6 +87,8 @@ const { test, run } = suite_with_variants<SSRTest, 'sync' | 'async', CompileOpti
 				csp: config.csp
 			});
 			rendered = is_async ? await render_result : render_result;
+			// we need to access these inside the try-catch otherwise errors in the script tag are not caught
+			({ body, head, hashes } = rendered);
 		} catch (error) {
 			errored = true;
 			if (config.error) {
@@ -97,8 +103,6 @@ const { test, run } = suite_with_variants<SSRTest, 'sync' | 'async', CompileOpti
 			assert.fail('Expected an error to be thrown, but rendering succeeded.');
 		}
 
-		const { body, head, hashes } = rendered;
-
 		fs.writeFileSync(
 			`${test_dir}/_output/${is_async ? 'async_rendered.html' : 'rendered.html'}`,
 			body