feat(bp): /battle_pass/buy — crystal-cost + retroactive premium grants

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,28 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.BattlePass;
+
+/// <summary>
+/// /battle_pass/buy response (Wizard/BattlePassBuyTask.cs:30-52). result_code carries the
+/// envelope failure path: 22 = insufficient crystals, 23 = already premium,
+/// 24 = outside BP period / season mismatch.
+/// </summary>
+[MessagePackObject]
+public class BattlePassBuyResponse
+{
+    [JsonPropertyName("result_code")]
+    [Key("result_code")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public int ResultCode { get; set; } = 1;
+
+    [JsonPropertyName("achieved_info")]
+    [Key("achieved_info")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public BattlePassAchievedInfoDto AchievedInfo { get; set; } = new();
+
+    [JsonPropertyName("reward_list")]
+    [Key("reward_list")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public List<BattlePassRewardListEntryDto> RewardList { get; set; } = new();
+}