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/BattlePassSalesPeriodInfoDto.cs

@@ -0,0 +1,16 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.BattlePass;
+
+/// <summary>
+/// sales_period_info on /battle_pass/item_list (Wizard/BattlePassPurchaseInfoTask.cs:26-29).
+/// Only sales_period_time is read by the client; other fields are unused but allowed.
+/// </summary>
+[MessagePackObject]
+public class BattlePassSalesPeriodInfoDto
+{
+    [JsonPropertyName("sales_period_time")]
+    [Key("sales_period_time")]
+    public string? SalesPeriodTime { get; set; }
+}