feat(screenshot): add webp output format

[yury-s]

Summary

• Adds webp to page.screenshot()/locator.screenshot() type union. Encoded natively via CDP (Chromium), Page.screenshot (Firefox), and Page.snapshotRect with format+quality (WebKit, which also drops the PNG→JPEG re-encode round-trip in Node).
• Adds webp to the MCP browser_take_screenshot tool.

Fixes microsoft/playwright-mcp#1632

[dgozman]

No default quality for webp?

[yury-s]

WebP on macOS WebKit is skipped.

The WebKit protocol patch (browser_patches/webkit via playwright-browsers#2281) is in revision 2302, but macOS's CGImageDestination does not advertise a webp encoder UTI. So Page.snapshotRect with format: webp returns data:, (empty) on macOS WebKit even though jpeg and png work fine.

Verified with this Swift script on macOS 26.5:

import CoreServices
import ImageIO
import UniformTypeIdentifiers

if let types = CGImageDestinationCopyTypeIdentifiers() as? [String] {
    print("Image destination UTIs:")
    for t in types where t.lowercased().contains("webp") || t.lowercased().contains("jpeg") || t.lowercased().contains("png") {
        print("  \(t)")
    }
}

Output:

Image destination UTIs:
  public.jpeg
  public.png
  public.jpeg-2000

No public.webp. The WebKit-side MIMETypeRegistry::isSupportedImageMIMETypeForEncoding for COCOA is populated from this list, so the encoder silently no-ops. Linux WebKit (Skia) does have a webp encoder, so those tests still run. Skipping the three webp tests for macOS WebKit until Apple ships a CG webp encoder (or WebKit builds with Skia on macOS).

[github-actions]

Test results for "MCP"

7290 passed, 1121 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

2 flaky

⚠️ [chromium-library] › library/browsertype-connect.spec.ts:699 › run-server › should record trace with sources `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/page-close.spec.ts:152 › should not treat navigations as new popups `@chromium-ubuntu-22.04-node24`

39525 passed, 775 skipped

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Merge workflow run.