fix(web-integration): handle serialized static screenshots

[quanru]

Summary

• Handle StaticPage screenshots that arrive as JSON-serialized ScreenshotItem objects from Playground Server mode.
• Preserve support for normal ScreenshotItem instances and string/base64 screenshots, while throwing a clearer error for screenshot refs without inline base64 data.
• Add focused StaticPage coverage and a Playground Server regression test for JSON-serialized screenshots.

Root Cause

The first introducing change was 8e02e9a292db73c2b9c29f24ffff9b1580433f11 (refactor(core): replace StorageProvider with ReportGenerator protocol, #1823) on 2026-02-03. That refactor removed the ScreenshotItem.toJSON() path that serialized screenshots as { base64 }. In report Playground Server mode, UIContext is sent through JSON to /execute; the server middleware then updates the static page context with a plain object containing internal screenshot fields such as _base64. StaticPage.screenshotBase64() only recognized a public base64 property and otherwise returned the whole object, which later reached imageInfoOfBase64() and produced imageBase64.replace is not a function.

9a84b5bfd6e8b8dee8c5f5f965453bf6c166fdfe (#1994) later kept the same fallback while removing legacy context compatibility, but it was not the first introducing commit for this error shape.

Validation

• pnpm --dir packages/web-integration exec vitest --run tests/unit-test/static-page.test.ts tests/unit-test/playground-server.test.ts
• npx nx test @midscene/web -- tests/unit-test/static-page.test.ts tests/unit-test/playground-server.test.ts
• npx nx build @midscene/web
• pnpm run lint
• git diff --check

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