Fix app staging header request guard (#63511)

Copilot-Session: 7c52b57f-2c29-4aa1-8233-99d0d6e13571

Author: heiskr

src/frame/pages/app.tsx

@@ -148,7 +148,7 @@ MyApp.getInitialProps = async (appContext: AppContext) => {
       }
     }
   }
-  const headerValue = req.headers['x-ong-external-url']
+  const headerValue = req?.headers['x-ong-external-url']
   const stagingName = (typeof headerValue === 'string' ? headerValue : headerValue?.[0])?.match(
     /staging-(\w+)\./,
   )?.[1]

src/frame/tests/app.ts

@@ -0,0 +1,57 @@
+import { IncomingMessage, type IncomingHttpHeaders } from 'node:http'
+import { Socket } from 'node:net'
+import type { AppContext } from 'next/app'
+import { describe, expect, test, vi } from 'vitest'
+
+import MyApp from '@/frame/pages/app'
+
+vi.mock('@primer/react', () => ({
+  ThemeProvider: ({ children }: { children: unknown }) => children,
+}))
+
+type TestRequest = NonNullable<AppContext['ctx']['req']> & {
+  context: Record<string, unknown>
+}
+
+function createRequest(headers: IncomingHttpHeaders): TestRequest {
+  const req = new IncomingMessage(new Socket()) as TestRequest
+  req.context = {}
+  req.headers = headers
+  return req
+}
+
+function createAppContext(req?: TestRequest): AppContext {
+  const Component = () => null
+
+  return {
+    AppTree: Component,
+    Component,
+    ctx: {
+      AppTree: Component,
+      pathname: '/',
+      query: {},
+      req,
+    },
+    router: {} as AppContext['router'],
+  }
+}
+
+describe('MyApp.getInitialProps', () => {
+  test('handles client-side error renders without a request', async () => {
+    const props = await MyApp.getInitialProps(createAppContext())
+
+    expect(props.stagingName).toBeUndefined()
+  })
+
+  test('sets the staging name from the request header', async () => {
+    const props = await MyApp.getInitialProps(
+      createAppContext(
+        createRequest({
+          'x-ong-external-url': 'https://docs-staging-cedar.example.com/',
+        }),
+      ),
+    )
+
+    expect(props.stagingName).toBe('cedar')
+  })
+})