Add pets for tiers 4-6.

internal/game/game_test.go

@@ -1,6 +1,7 @@
 package game
 
 import (
+	"slices"
 	"testing"
 )
 
@@ -171,7 +172,8 @@ func TestTradeInThreeMatchingSuits(t *testing.T) {
 	if err := g.TradeChoose(p.ID, 0); err != nil {
 		t.Fatal(err)
 	}
-	if len(p.Deck) != 1 || p.Deck[0].ID != chosen.ID {
+	// The chosen card joins the deck (its Buy effect may add apples too).
+	if slices.IndexFunc(p.Deck, func(c Card) bool { return c.ID == chosen.ID }) < 0 {
 		t.Fatal("chosen card should join the deck")
 	}
 	deck2 := g.ShopDecks[1]