fix(browser): dismiss a page's beforeunload prompt instead of wedging the session

[Benjamin-eecs]

Description

A page that arms beforeunload can wedge a browser session: the next navigation opens the native "Leave site?" prompt, nothing in the CLI can answer it (the daemon owns the CDP session, so browser dialog dismiss reports no dialog), navigation never returns, and every later evaluate reads an empty document. Any adapter that clicks before navigating can supply the user gesture the prompt needs; twitter post wedged a session for me twice.

Both attach paths, the extension and the direct-CDP CDPBridge, install two halves: a guard registered through Page.addScriptToEvaluateOnNewDocument, early enough to stop the site's own handler, and a Page.javascriptDialogOpening handler that accepts only beforeunload dialogs, covering documents already loaded at attach. alert, confirm and prompt stay with browser dialog.

Related issue: none filed; #1972 is @biospark33's twitter-scoped PR for the same wedge, and this installs the guard for every adapter in the layer that runs before page scripts.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

Navigation-level A/B on Chrome 151.0.7922.76 over raw CDP: a page arms beforeunload at load, receives a real click for the user gesture, then navigates away. Without the guard the prompt opens and the navigation never completes, which is the wedge; with the guard pre-injected the navigation completes with no dialog:

UNGUARDED {"dialogOpened":true,"dialogType":"beforeunload","navigationCompleted":false}
GUARDED   {"dialogOpened":false,"dialogType":null,"navigationCompleted":true}

npx vitest run src/browser 433 / 433 and the extension suite 99 / 99, with tests for the guard, the injection at attach, the CDPBridge dialog accept, keeping the two guard copies in step, and leaving non-beforeunload dialogs alone. Typecheck, both lint gates and doc coverage pass. extension/dist/background.js is rebuilt, as the repo tracks it.

Not verified end to end through a loaded build of this extension, since the daemon tracks one extension connection at a time and the installed build holds the slot.

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