fix browser session lease cleanup

[ruohuaqian]

Summary

• add an opt-in BrowserBridge close path that releases the active page/session lease
• thread releasePageOnClose through browserSession
• enable the release fallback for one-shot adapter runs while preserving --keep-tab and persistent site sessions

Root Cause

BrowserBridge.close() only dropped its local Page reference. If an adapter process ended after the explicit page cleanup path was skipped or interrupted, the browser bridge daemon could keep the session lease alive until idle cleanup, leaving Chrome tab groups around.

Validation

• npm run typecheck
• npx vitest run --project unit src/browser.test.ts
• npx vitest run --project unit src/execution.test.ts