Missing Preview Image for Link Embeds in Arcprompt

- App/Feature: Arcprompt - Link Previews / Rich Embeds
- Issue: YouTube (and potentially other) link previews display placeholder image instead of actual thumbnail.
- Description: When a YouTube link is shared or generated in the conversation, the rich preview card displays a generic placeholder image (gray background with mountain icon) rather than the actual video thumbnail/preview image. The link metadata (title, URL, description) loads correctly, but the visual preview image fails to render.
- Expected Behavior: The preview card should display the actual YouTube video thumbnail or relevant preview image from the linked content.
- Actual Behavior: Only a generic placeholder image is shown in the preview card, making it difficult to visually identify the content before clicking.
- Example URL: https://m.youtube.com/results?search_query=Math+Antics+multiplication (or similar YouTube links)
- Screen: Conversation view with YouTube link embed/preview card.