feat(webkit): throw on webp screenshots on macOS

CG on macOS does not advertise a webp encoder UTI, so requesting
webp would silently produce an empty buffer. Surface this clearly
to users instead of letting it look like a successful no-op.

Author: yury-s

packages/playwright-core/src/server/webkit/wkPage.ts

@@ -868,6 +868,8 @@ export class WKPage implements PageDelegate {
   }
 
   async takeScreenshot(progress: Progress, format: string, documentRect: types.Rect | undefined, viewportRect: types.Rect | undefined, quality: number | undefined, fitsViewport: boolean, scale: 'css' | 'device'): Promise<Buffer> {
+    if (format === 'webp' && process.platform === 'darwin')
+      throw new Error('webp screenshots are not supported in WebKit on macOS');
     const rect = (documentRect || viewportRect)!;
     const omitDeviceScaleFactor = scale === 'css';
     this.validateScreenshotDimension(rect.width, omitDeviceScaleFactor);