feat(screenshot): add full-page screenshot support for iframes

[absarasw]

Add ability to capture full-page screenshots of iframe content by
temporarily expanding the iframe to reveal all scrollable content.

Changes:

• Add iframeUid parameter to explicitly target an iframe for capture
• Auto-detect main content iframe when fullPage=true is used
• Temporarily expand iframe dimensions to capture full scrollable content
• Restore original iframe styles after screenshot capture
• Add proper element handle disposal for better memory management

This enables capturing complete iframe content (like embedded editors or
content frames) that would otherwise be clipped to the visible viewport.

[google-cla]

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