fix: inline VS Code webviews in snapshots

[zhanba]

Summary

• track VS Code webview iframe targets so their sessions are available to snapshotting
• resolve the nested active-frame inside VS Code webviews when taking snapshots
• reuse the same frame lookup helper in frame selection and add focused VS Code regression coverage

Testing

• cargo test vscode --manifest-path cli/Cargo.toml
• cargo fmt --manifest-path cli/Cargo.toml -- --check

[vercel]

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[zhanba]

@ctate would you consider this pr? it is important as VSCode extension api forces all extensions to use this two level nested iframe structure, and agent-browser can't work with it properly.