feat(svc): IArenaTwoPickService + response DTOs + GetTopAsync

6 response DTOs, IArenaTwoPickService interface + ArenaTwoPickException,
ArenaTwoPickService skeleton with GetTopAsync implemented and stubs for
Tasks 13-15. 3 NUnit tests for GetTopAsync all pass. DI: AddScoped.

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaTwoPick/FinishResponseDto.cs

@@ -0,0 +1,17 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Common.ArenaTwoPick;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses.ArenaTwoPick;
+
+[MessagePackObject]
+public class FinishResponseDto
+{
+    /// <summary>Per-grant deltas — drives "+N received" popup.</summary>
+    [JsonPropertyName("rewards")] [Key("rewards")]
+    public List<RewardEntryDto> Rewards { get; set; } = new();
+
+    /// <summary>Post-state totals — drives PlayerStaticData.UpdateHaveUserGoodsNumByJsonData.</summary>
+    [JsonPropertyName("reward_list")] [Key("reward_list")]
+    public List<RewardEntryDto> RewardList { get; set; } = new();
+}