Document JavaScript top-level await support

Add documentation for new top-level await feature in JavaScript contexts:
- No IIFE wrapper needed for async code
- Last expression automatically returned as result
- Seamless support for multiple await expressions

Updated:
- API reference (interpreter.mdx) with top-level await example
- How-to guide (code-execution.mdx) with comprehensive examples

Related to cloudflare/sandbox-sdk#261

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: github-actions[bot]

src/content/docs/sandbox/api/interpreter.mdx

@@ -9,6 +9,8 @@ import { TypeScriptExample } from "~/components";
 
 Execute Python, JavaScript, and TypeScript code with support for data visualizations, tables, and rich output formats. Contexts maintain state (variables, imports, functions) across executions.
 
+JavaScript contexts support top-level `await` - no need to wrap async code in IIFE. The last expression is automatically returned as the result.
+
 ## Methods
 
 ### `createCodeContext()`
@@ -91,6 +93,23 @@ console.log(result.results[0].text); // "78.53981633974483"
 ```
 </TypeScriptExample>
 
+**JavaScript with top-level await**:
+
+<TypeScriptExample>
+```
+const ctx = await sandbox.createCodeContext({ language: 'javascript' });
+
+// Top-level await - no IIFE needed!
+const result = await sandbox.runCode(`
+const response = await fetch('https://api.example.com/data');
+const data = await response.json();
+data.result
+`, { context: ctx });
+
+console.log(result.results[0]); // Last expression is automatically returned
+```
+</TypeScriptExample>
+
 :::note[Default context behavior]
 If no `context` is provided, a default context is automatically created/reused for the specified `language`. While convenient for quick tests, **explicitly creating contexts is recommended** for production use to maintain predictable state.
 

src/content/docs/sandbox/guides/code-execution.mdx

@@ -51,6 +51,10 @@ const jsContext = await sandbox.createCodeContext({
 ```
 </TypeScriptExample>
 
+:::note[JavaScript top-level await]
+JavaScript contexts automatically support top-level `await` without wrapping code in async IIFE. The last expression value is returned as the result.
+:::
+
 ## Execute code
 
 ### Simple execution
@@ -133,6 +137,36 @@ console.log(result.output); // "Mean: 3.0"
 Context state is lost if the container restarts due to inactivity. For critical data, store results outside the sandbox or design your code to reinitialize as needed.
 :::
 
+### Top-level await (JavaScript)
+
+JavaScript contexts support top-level `await` without IIFE wrappers. The last expression is automatically returned:
+
+<TypeScriptExample>
+```
+const jsContext = await sandbox.createCodeContext({
+  language: 'javascript'
+});
+
+// No IIFE needed - await works at top level
+const result = await sandbox.runCode(`
+const data = await Promise.resolve([1, 2, 3, 4, 5]);
+const sum = data.reduce((a, b) => a + b, 0);
+sum
+`, { context: jsContext.id });
+
+console.log(result.results[0].text); // "15"
+
+// Multiple awaits work seamlessly
+const result2 = await sandbox.runCode(`
+const a = await Promise.resolve(10);
+const b = await Promise.resolve(20);
+a + b
+`, { context: jsContext.id });
+
+console.log(result2.results[0].text); // "30"
+```
+</TypeScriptExample>
+
 ## Handle rich outputs
 
 The code interpreter returns multiple output formats: