Document async/await support in JS/TS code execution

Add documentation for automatic Promise resolution in JavaScript and
TypeScript code contexts. This clarifies the behavior fixed in PR #246
where async functions and Promise.resolve() now correctly return their
resolved values instead of empty objects.

Changes:
- Add async code execution examples to API reference
- Add async error handling examples
- Add practical async/await guide section
- Document that Promises are automatically awaited

Related: cloudflare/sandbox-sdk#246

Author: github-actions[bot]

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

@@ -90,6 +90,40 @@ console.log(result.results[0].text); // "15"
 </TypeScriptExample>
 :::
 
+**Async code execution (JavaScript/TypeScript)**:
+
+JavaScript and TypeScript contexts automatically await Promise results:
+
+<TypeScriptExample>
+```
+const ctx = await sandbox.createCodeContext({ language: 'javascript' });
+
+// Async IIFE - returns resolved value
+const result1 = await sandbox.runCode(
+  '(async () => { return 42; })()',
+  { context: ctx }
+);
+console.log(result1.results[0].json); // 42
+
+// Promise.resolve - returns resolved value
+const result2 = await sandbox.runCode(
+  'Promise.resolve({ status: "success", value: 123 })',
+  { context: ctx }
+);
+console.log(result2.results[0].json); // { status: "success", value: 123 }
+
+// Nested async operations
+const result3 = await sandbox.runCode(`
+(async () => {
+  const a = await Promise.resolve(10);
+  const b = await Promise.resolve(20);
+  return a + b;
+})()
+`, { context: ctx });
+console.log(result3.results[0].json); // 30
+```
+</TypeScriptExample>
+
 **Error handling**:
 
 <TypeScriptExample>
@@ -104,6 +138,25 @@ if (result.error) {
 ```
 </TypeScriptExample>
 
+**Async error handling (JavaScript/TypeScript)**:
+
+<TypeScriptExample>
+```
+const ctx = await sandbox.createCodeContext({ language: 'javascript' });
+
+// Rejected promises are caught as errors
+const result = await sandbox.runCode(
+  'Promise.reject(new Error("Failed"))',
+  { context: ctx }
+);
+
+if (result.error) {
+  console.error(result.error.message); // Error details
+  console.error(result.logs.stderr);   // Stack trace
+}
+```
+</TypeScriptExample>
+
 ### `listCodeContexts()`
 
 List all active code execution contexts.

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

@@ -183,6 +183,51 @@ print("Done!")
 ```
 </TypeScriptExample>
 
+## Execute async JavaScript/TypeScript
+
+JavaScript and TypeScript contexts automatically await Promise results, making async code execution seamless:
+
+<TypeScriptExample>
+```
+const context = await sandbox.createCodeContext({
+  language: 'javascript'
+});
+
+// Execute async code that fetches data
+const result = await sandbox.runCode(`
+(async () => {
+  // Simulate async data fetching
+  const fetchData = (id) => Promise.resolve({ id, name: 'Item ' + id });
+
+  const item1 = await fetchData(1);
+  const item2 = await fetchData(2);
+
+  return [item1, item2];
+})()
+`, { context: context.id });
+
+console.log('Results:', result.results[0].json);
+// Results: [{ id: 1, name: 'Item 1' }, { id: 2, name: 'Item 2' }]
+```
+</TypeScriptExample>
+
+**Error handling for async code**:
+
+<TypeScriptExample>
+```
+const result = await sandbox.runCode(`
+(async () => {
+  throw new Error('Async operation failed');
+})()
+`, { context: context.id });
+
+if (result.error) {
+  console.error('Error:', result.error.message);
+  console.error('Stack:', result.logs.stderr.join('\n'));
+}
+```
+</TypeScriptExample>
+
 ## Execute AI-generated code
 
 Run LLM-generated code safely in a sandbox: