Creative tab selection test: give the category tab a stack

The fixture's CATEGORY tab was empty, and an empty category tab is now
hidden (CreativeModeTab.shouldDisplay), so tab 0 was no longer the tab the
test selects. It now holds a stone stack.

Part of #3393.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-client/src/game.rs

@@ -16318,7 +16318,10 @@ mod tests {
     fn selecting_creative_tabs_does_not_drop_the_carried_stack_on_release() {
         let mut creative = Creative::from_json(&serde_json::json!({
             "schema": 2,
-            "tabs": [{"type": "CATEGORY"}, {"type": "INVENTORY"}]
+            "tabs": [
+                {"type": "CATEGORY", "items": ["{count:1,id:\"minecraft:stone\"}"]},
+                {"type": "INVENTORY"}
+            ]
         }))
         .unwrap();
         for button in [Button::Left, Button::Right] {