fix(ui): Use assetUrl as fallback for image thumbnail

This change adds `assetUrl` as a fallback option when determining the image URL for a thumbnail.

The priority is now `thumbUrl`, then `imageUrl`, and finally `assetUrl`. This ensures that an image thumbnail can still be displayed even if the `thumbUrl` and `imageUrl` are not available, by using the local asset URL as a last resort.

Author: xsahil03x

packages/stream_chat_flutter/lib/src/attachment/thumbnail/image_attachment_thumbnail.dart

@@ -90,7 +90,8 @@ class StreamImageAttachmentThumbnail extends StatelessWidget {
 
         // If the remote image URL is available, we can directly show it using
         // the _RemoteImageAttachment widget.
-        if (image.thumbUrl ?? image.imageUrl case final imageUrl?) {
+        final imageUrl = image.thumbUrl ?? image.imageUrl ?? image.assetUrl;
+        if (imageUrl case final imageUrl?) {
           var resizedImageUrl = imageUrl;
           if (effectiveThumbnailSize case final thumbnailSize?) {
             resizedImageUrl = imageUrl.getResizedImageUrl(