feat: Set summary thumbnail path for WebXDCs to "webxdc-icon://last-msg-id" (#5782)

This is a hint for apps that a WebXDC icon should be shown in the summary, e.g. in the
chatlist. Otherwise it's not clear when it should be shown, e.g. it shouldn't be shown in a reaction
summary.

src/webxdc.rs

@@ -2650,6 +2650,7 @@ sth_for_the = "future""#
         assert_eq!(chatlist.len(), 1);
         let summary = chatlist.get_summary(&t, 0, None).await?;
         assert_eq!(summary.text, "nice app!".to_string());
+        assert_eq!(summary.thumbnail_path.unwrap(), "webxdc-icon://last-msg-id");
 
         Ok(())
     }