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/Controllers/ArenaController.cs

@@ -43,6 +43,14 @@ public class ArenaController : SVSimController
             catch { /* fall back to defaults */ }
         }
 
+        // Default Challenge Master reward steps from prod capture: 3 milestones at 5/10/15 wins.
+        var rewardSteps = new Dictionary<string, string>
+        {
+            ["5"]  = "5",
+            ["10"] = "10",
+            ["15"] = "15",
+        };
+
         return Ok(new GetChallengeInfoResponseDto
         {
             ChallengeName = name,
@@ -51,9 +59,17 @@ public class ArenaController : SVSimController
             TwoPickAllWinCount = 0,
             RewardStepInfo = new RewardStepInfoDto
             {
-                MaxRewardStep = 0,
-                RewardStepList = new List<int>(),
+                MaxRewardStep = 15,
+                RewardStepList = rewardSteps,
             },
         });
     }
+
+    [HttpPost("get_challenge_ranking_history")]
+    public IActionResult GetChallengeRankingHistory([FromBody] GetChallengeInfoRequest req)
+    {
+        if (!TryGetViewerId(out _)) return Unauthorized();
+        // Prod returns {two_pick: [], sealed: []}. Stub matches.
+        return Ok(new GetChallengeRankingHistoryResponseDto());
+    }
 }