Fix process_frame() returning invalid corsIframes entries when iframe element is not found

[Copilot]

process_frame() was returning a result even when the matching <iframe> element wasn't found on the main page, producing corsIframes entries with a null percyElementId that the CLI cannot merge back into the DOM snapshot.

Changes

• percy/screenshot.py: Added an early-return guard after the page.evaluate() call — if iframe_data is None or lacks percyElementId, the function returns None and logs at debug level instead of propagating an incomplete entry

if not iframe_data or not iframe_data.get("percyElementId"):
    log(
        f"Skipping cross-origin frame {frame_url}: "
        "no matching iframe element with percyElementId found on main page",
        "debug"
    )
    return None

• tests/test_screenshot.py: Added two new unit tests covering the None iframe_data case and the missing percyElementId case

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