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/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();
+}