fix(chromium): preserve main frame on unknown iframe parent

[SivaShankar A V (sivashankarav)]

Summary

• Skip attaching an iframe target when its parent frame is not registered during CDP connection, rather than replacing the page's main frame.
• Add a regression test connecting to a preloaded page with nested cross-site and sandboxed iframes.

Fixes #42955

Test plan

• CRPATH=/usr/bin/google-chrome npm run ctest -- --project=chromium-library tests/library/chromium/connect-over-cdp.spec.ts -g 'should not replace the main frame' --repeat-each=3 --reporter=line (3 passed)
• Existing CDP iframe and page-title controls passed.
• TypeScript and ESLint checks timed out in this environment.

[Dmitry Gozman (dgozman)]

Closing in favor of #43000.