3591832: useSyncIframeHeightToContent: vh-height sections with background images cause unbounded iframe growth via feedback loop

AI-Generated: Yes (Used opencode to fix the bug).

Testing instructions

• Create a code component with this structure on 1.x:

const sizeClass = { default: 'h-[60vh]', short: 'h-[40vh]' };

export default function VhImageHero({ size = 'default' }) {
  const heightClass = sizeClass[size] || sizeClass.default;
  return (
    <section
      className={`relative flex w-full items-center justify-center overflow-hidden ${heightClass}`}
    >
      <div className="absolute inset-0">
        <img
          src="https://images.unsplash.com/photo-1542314831-068cd1dbfeeb?auto=format&fit=crop&w=1920&q=80"
          alt="Hero background"
          className="h-full w-full object-cover"
        />
      </div>
    </section>
  );
}

• Add the component to a Canvas page in the editor.
• The background image takes up a disproportionate amount of vertical space.
• Apply fix in this MR to resolve the issue.

Closes #3591832