feat(bp): /battle_pass/item_list — derives product per active season

Adds BattlePassSalesPeriodInfoDto, BattlePassProductDto, BattlePassItemListResponse DTOs,
GetItemListAsync on BattlePassService (one product if not premium + CanPurchase, empty if
already premium or off-season), and the /battle_pass/item_list controller action.
2 new integration tests; all 408 pass.

SVSim.EmulatedEntrypoint/Models/Dtos/BattlePass/BattlePassItemListResponse.cs

@@ -0,0 +1,25 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.BattlePass;
+
+/// <summary>
+/// /battle_pass/item_list response (Wizard/BattlePassPurchaseInfoTask.cs:23-44).
+/// </summary>
+[MessagePackObject]
+public class BattlePassItemListResponse
+{
+    [JsonPropertyName("premium_pass_description")]
+    [Key("premium_pass_description")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public string PremiumPassDescription { get; set; } = "";
+
+    [JsonPropertyName("sales_period_info")]
+    [Key("sales_period_info")]
+    public BattlePassSalesPeriodInfoDto? SalesPeriodInfo { get; set; }
+
+    [JsonPropertyName("products")]
+    [Key("products")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public List<BattlePassProductDto> Products { get; set; } = new();
+}