Preserve new tab semantics for snapshot preload

[sjmiller609]

Summary

• initiate the snapshot preload through chrome://newtab/ so Chromium applies the managed external new-tab policy
• continue waiting for DuckDuckGo to finish loading, including navigation retries
• preserve the empty new-tab omnibox instead of showing the destination URL as a normal navigation

Testing

• cd server && go vet ./...
• cd server && go test -race $(go list ./... | grep -v /e2e$)
• cd server && make build
• manually verified Chromium loads an external managed new-tab page when launched through chrome://newtab/

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Note

Low Risk
Scoped to snapshot preload CDP navigation and its helper; no auth or data-path changes, with behavior covered by updated unit tests.

Overview
Snapshot preload no longer CDP-navigates straight to the DuckDuckGo start URL. DispatchStartURLAndWait now takes a separate navigationURL and destination: CDP issues Page.navigate to the navigation URL, while readiness still waits for the destination host to reach complete.

prepareSnapshotStartPage uses chrome://newtab/ as the navigation URL and the existing start page as the destination so Chromium applies the managed external new-tab flow instead of treating the load as a normal navigation (preserving an empty omnibox). Chrome error-page retries re-issue navigation to navigationURL, not the destination.

Tests assert the initial and retry navigations target chrome://newtab/.

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