Secure popup image URL validation in UIManager and BabylonSceneManager

[brettfxio]

Enforced secure asset URL validation for popup images during creation and modification in UIManager.createPopup and BabylonSceneManager.setPopupImage. Restored the popup security integration test suite tests/popup_security.spec.js to verify the security improvements under various HTTP/HTTPS scenarios.

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PR created automatically by Jules for task 11045006114676245100 started by @brettfxio

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