[Feature Request] Support cross-document tool evaluation

[Kulikowski]

Currently, the webmcp-tools evaluation suite lacks a way to verify tool outputs that happen cross-document.

Proposed Change

Add support for the following API within the evaluation environment:
navigator.modelContextTesting.getCrossDocumentScriptToolResult()

[swissspidy]

Context for myself:

webmachinelearning/webmcp#76
https://chromium-review.googlesource.com/c/chromium/src/+/7514110

Basically, when I do navigator.modelContextTesting.executeTool(...) where the tool performs a navigation (such as a declarative tool definition on a <form> that triggers form submission), then I have to use getCrossDocumentScriptToolResult() to get the tool result.
The "result" in that case is just whatever is in <script type="application/ld+json"> elements on that page

[beaufortfrancois]

See https://chromium-review.git.corp.google.com/c/chromium/src/+/7511354 as well