fix: ensure iframe cleanup in getUntaintedPrototype on error or early return

[heathdutton]

Fixes #1627

The getUntaintedPrototype function creates a temporary iframe to get clean prototypes, but fails to remove it when:

• contentWindow is null (early return)
• An exception is thrown

This adds a finally block to ensure the iframe is always cleaned up.

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Latest commit: b64b625

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[Copilot]

Pull request overview

This PR fixes a resource leak where temporary iframes created to access untainted prototypes were not always properly cleaned up. The fix ensures the iframe is removed from the DOM in all exit paths.

Key Changes:

• Moved iframe variable declaration outside try block to make it accessible in finally block
• Replaced inline cleanup with a finally block that executes regardless of how the function exits
• Added safety check for parentNode existence before attempting removal

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