Document generic types for extending Sandbox class

Update API documentation to reflect TypeScript generic type parameters
added to getSandbox(), SandboxEnv, and proxyToSandbox(). These changes
fix type errors when extending the Sandbox class with custom methods.

Changes:
- Update getSandbox() signature to show generic type parameter
- Add section on extending Sandbox class with example
- Document proxyToSandbox() function with generic types
- Document SandboxEnv type helper for custom Sandbox classes

Related to cloudflare/sandbox-sdk#264

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: github-actions[bot]

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

@@ -16,11 +16,11 @@ Create and manage sandbox containers. Get sandbox instances, configure options,
 Get or create a sandbox instance by ID.
 
 ```ts
-const sandbox = getSandbox(
-  binding: DurableObjectNamespace<Sandbox>,
+function getSandbox<T extends Sandbox<any>>(
+  binding: DurableObjectNamespace<T>,
   sandboxId: string,
   options?: SandboxOptions
-): Sandbox
+): T
 ```
 
 **Parameters**:
@@ -32,7 +32,10 @@ const sandbox = getSandbox(
   - `containerTimeouts` - Configure container startup timeouts
   - `normalizeId` - Lowercase sandbox IDs for preview URL compatibility (default: `false`)
 
-**Returns**: `Sandbox` instance
+**Returns**: Sandbox instance (type `T`, which defaults to `Sandbox`)
+
+**Type parameter**:
+- `T extends Sandbox<any>` - The Sandbox class type. Use this when extending the Sandbox class with custom methods to get proper type inference.
 
 :::note
 The container starts lazily on first operation. Calling `getSandbox()` returns immediately—the container only spins up when you execute a command, write a file, or perform other operations. See [Sandbox lifecycle](/sandbox/concepts/sandboxes/) for details.
@@ -56,6 +59,40 @@ export default {
 When using `keepAlive: true`, you **must** call `destroy()` when finished to prevent containers running indefinitely.
 :::
 
+#### Extending the Sandbox class
+
+You can extend the Sandbox class to add custom methods. The generic type parameter ensures proper type inference:
+
+<TypeScriptExample>
+```
+import { getSandbox, Sandbox, type SandboxEnv } from '@cloudflare/sandbox';
+
+// Define your extended Sandbox class
+export class CustomSandbox extends Sandbox<CustomSandbox> {
+  async runTests(): Promise<{ passed: number; failed: number }> {
+    const result = await this.exec('npm test');
+    // Parse test output
+    return { passed: 10, failed: 0 };
+  }
+}
+
+// Export your custom class as the Durable Object
+export { CustomSandbox as Sandbox };
+
+// Define environment with your custom class
+type Env = SandboxEnv<CustomSandbox>;
+
+export default {
+  async fetch(request: Request, env: Env): Promise<Response> {
+    // getSandbox now returns CustomSandbox type with runTests() available
+    const sandbox = getSandbox(env.Sandbox, 'my-sandbox');
+    const testResults = await sandbox.runTests();
+    return Response.json(testResults);
+  }
+};
+```
+</TypeScriptExample>
+
 ---
 
 ### `destroy()`

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

@@ -134,6 +134,55 @@ export default {
 ```
 </TypeScriptExample>
 
+### `proxyToSandbox()`
+
+Route incoming requests to exposed sandbox ports via preview URLs. This function must be called first in your Worker's fetch handler to enable preview URL routing.
+
+```ts
+function proxyToSandbox<
+  T extends Sandbox<any>,
+  E extends SandboxEnv<T>
+>(request: Request, env: E): Promise<Response | null>
+```
+
+**Parameters**:
+
+- `request` - Incoming request to potentially route
+- `env` - Worker environment binding with Sandbox namespace
+
+**Returns**: `Promise<Response | null>` - Response if the request matches a preview URL pattern, or `null` if not a preview URL request
+
+**Type parameters**:
+- `T extends Sandbox<any>` - The Sandbox class type (useful when extending Sandbox)
+- `E extends SandboxEnv<T>` - Environment type with Sandbox namespace binding
+
+<TypeScriptExample>
+```ts
+import { getSandbox, proxyToSandbox, type SandboxEnv } from "@cloudflare/sandbox";
+
+export { Sandbox } from "@cloudflare/sandbox";
+
+type Env = SandboxEnv;
+
+export default {
+  async fetch(request: Request, env: Env): Promise<Response> {
+    // Route preview URL requests first
+    const proxyResponse = await proxyToSandbox(request, env);
+    if (proxyResponse) return proxyResponse;
+
+    // Your application routes
+    const sandbox = getSandbox(env.Sandbox, 'my-sandbox');
+    // ...
+    return new Response('OK');
+  }
+};
+```
+</TypeScriptExample>
+
+:::note
+When extending the Sandbox class, use the type parameters to maintain type safety. See [Extending the Sandbox class](/sandbox/api/lifecycle/#extending-the-sandbox-class) for examples.
+:::
+
 ## Related resources
 
 - [Preview URLs concept](/sandbox/concepts/preview-urls/) - How preview URLs work