fix(arena): match prod get_challenge_info wire shape; stub ranking_history

Prod /arena/get_challenge_info capture (Season 26):
- reward_step_info.reward_step_list is a Dict<string,string>
  ({"5":"5","10":"10","15":"15"}), not the List<int> I'd assumed
- max_reward_step is stringified

The previous stub would have parsed at the client (LitJson tolerates the
shape via indexed iteration), but cleaning to match prod exactly.

Also stubs /arena/get_challenge_ranking_history (new endpoint observed
in the same capture). Prod ships {two_pick: [], sealed: []} with no
history populated — empty lists match. Routing smoke added.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/Arena/GetChallengeInfoResponseDto.cs

@@ -1,5 +1,6 @@
 using System.Text.Json.Serialization;
 using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Common;
 
 namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses.Arena;
 
@@ -32,9 +33,15 @@ public class GetChallengeInfoResponseDto
 [MessagePackObject]
 public class RewardStepInfoDto
 {
-    [JsonPropertyName("max_reward_step")] [Key("max_reward_step")]
+    [JsonPropertyName("max_reward_step")] [JsonConverter(typeof(StringifiedIntConverter))] [Key("max_reward_step")]
     public int MaxRewardStep { get; set; }
 
+    /// <summary>
+    /// Wire shape: dict keyed by stringified-int win count → stringified-int reward step
+    /// (prod capture: <c>{"5":"5","10":"10","15":"15"}</c>). Client parser at
+    /// ChallangeHistoryInfoTask.cs:43 iterates by Count + indexed value access, which works
+    /// for both arrays and LitJson object-iteration order — but prod always ships the dict.
+    /// </summary>
     [JsonPropertyName("reward_step_list")] [Key("reward_step_list")]
-    public List<int> RewardStepList { get; set; } = new();
+    public Dictionary<string, string> RewardStepList { get; set; } = new();
 }

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/Arena/GetChallengeRankingHistoryResponseDto.cs

@@ -0,0 +1,19 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses.Arena;
+
+/// <summary>
+/// Wire shape for /arena/get_challenge_ranking_history. Prod returns two empty lists
+/// (two_pick + sealed) per the season-26 capture. Populated history is per-viewer + per-season
+/// ranking snapshots; not tracked locally yet.
+/// </summary>
+[MessagePackObject]
+public class GetChallengeRankingHistoryResponseDto
+{
+    [JsonPropertyName("two_pick")] [Key("two_pick")]
+    public List<object> TwoPick { get; set; } = new();
+
+    [JsonPropertyName("sealed")] [Key("sealed")]
+    public List<object> Sealed { get; set; } = new();
+}