[Web Golden Testing] Support local golden testing and DPR alignment in flutter_tools and framework #191371

Part of #191313

Context

In #191314 (resolved by #191318), we implemented local screenshot and golden baseline diffing for Web Engine unit tests (felt test) using Skia Gold's REST API and in-memory pixel diffing.

Framework web tests run via flutter test --platform=chrome, which executes through packages/flutter_tools. We need to bring parity to flutter_tools and framework web golden testing as a separate follow-up step.

Problems & Requirements

1. Device Pixel Ratio (DPR) Alignment ([web] Flutter test suite in the web needs love. #143124):
   • In packages/flutter_tools/lib/src/web/chrome.dart, headless Chrome starts without explicit device scale factor flags, defaulting to DPR = 1.0, while packages/flutter_test/lib/src/_test_selector_web.dart overrides DPR to 3.0 in web test environments.
   • Update packages/flutter_tools to pass --force-device-scale-factor and ensure browser device emulation matches framework expectations.
2. Skia Gold REST Baselines for Framework Goldens:
   • Update package:flutter_goldens / flutter_goldens_web to retrieve baselines from Skia Gold's REST API and perform local comparison.
3. Re-enable & Validate Skipped Tests ([web] skipped image tests failing gold checks when enabled. #87933):
   • Unskip and validate packages/flutter/test/widgets/image_test.dart golden tests under the new workflow.

Related Issues

• [web] Flutter test suite in the web needs love. #143124: [web] Flutter test suite in the web needs love
• [web] skipped image tests failing gold checks when enabled. #87933: [web] skipped image tests failing gold checks when enabled
• [Web Engine] Support local screenshot and golden testing in felt test #191314: [Web Engine] Support local screenshot and golden testing in felt test (Predecessor)