fix: add screenshot mock to executor.test.ts for IPage compat

tsc --noEmit failed because createMockPage() was missing the
screenshot() method added to IPage in the round 2 review fix.

src/pipeline/executor.test.ts

@@ -26,6 +26,7 @@ function createMockPage(overrides: Partial<IPage> = {}): IPage {
     autoScroll: vi.fn(),
     installInterceptor: vi.fn(),
     getInterceptedRequests: vi.fn().mockResolvedValue([]),
+    screenshot: vi.fn().mockResolvedValue(''),
     ...overrides,
   };
 }