Function image_exists_in_unified_storage 

pub(crate) async fn image_exists_in_unified_storage(
    store: &Storage,
    imgref: &ImageReference,
) -> Result<bool>

Check whether the image exists in bootc’s unified container storage.

This is used for auto-detection: if the image already exists in bootc storage (e.g., from a previous bootc image set-unified or LBI pull), we can use the unified storage path for faster imports.

Returns true if the image exists in bootc storage.