For some websites, we get this error when attempting to grab a screenshot:

ERROR    [cdp_use.client] CDP Error for request 49: {'code': -32000, 'message': 'Command can only be executed on top-level targets'}                                                                                                                                                                                                                                                          

This PR fixes this by falling back to taking a screenshot of the top-level target.

Summary by cubic

Fixes screenshot failures by retrying against the top-level CDP target when the page returns "Command can only be executed on top-level targets." This makes screenshot capture reliable on sites with nested targets or iframes.

• Bug Fixes
  • Retry once using a confirmed top-level page session when the specific CDP error is hit.
  • Added a helper to select a top-level CDP session from agent focus or the session pool.
  • Better error wrapping and logging; highlight cleanup still runs on exit.