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/BattlePassBuyRequest.cs

@@ -0,0 +1,22 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.BattlePass;
+
+/// <summary>
+/// /battle_pass/buy request (Wizard/BattlePassBuyTask.cs:8-13). Inherits viewer_id, steam_id,
+/// steam_session_ticket from BaseRequest. Per memory feedback_msgpack_request_dtos, the
+/// [MessagePackObject] + [Key] attrs are required even though integration tests post JSON.
+/// </summary>
+[MessagePackObject]
+public class BattlePassBuyRequest : BaseRequest
+{
+    [JsonPropertyName("season_id")]
+    [Key("season_id")]
+    public int SeasonId { get; set; }
+
+    [JsonPropertyName("id")]
+    [Key("id")]
+    public int Id { get; set; }
+}