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();
+}

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

@@ -0,0 +1,43 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.BattlePass;
+
+/// <summary>
+/// One product in /battle_pass/item_list.products[] (Wizard/BattlePassPurchaseInfoTask.cs:32-43).
+/// Numerics on this DTO are numeric on the wire (the client uses .ToInt() and the captured
+/// shape isn't string-typed here).
+/// </summary>
+[MessagePackObject]
+public class BattlePassProductDto
+{
+    [JsonPropertyName("id")]
+    [Key("id")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public int Id { get; set; }
+
+    [JsonPropertyName("season_id")]
+    [Key("season_id")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public int SeasonId { get; set; }
+
+    [JsonPropertyName("name")]
+    [Key("name")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public string Name { get; set; } = "";
+
+    [JsonPropertyName("price_crystal")]
+    [Key("price_crystal")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public int PriceCrystal { get; set; }
+
+    [JsonPropertyName("description")]
+    [Key("description")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public string Description { get; set; } = "";
+
+    [JsonPropertyName("sales_period_info")]
+    [Key("sales_period_info")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public BattlePassSalesPeriodInfoDto SalesPeriodInfo { get; set; } = new();
+}

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; }
+}