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.UnitTests/RoutingSmokeTests.cs

@@ -110,6 +110,7 @@ public class RoutingSmokeTests
     [TestCase("/arena_two_pick_battle/finish")]
     [TestCase("/arena_colosseum/get_fee_info")]
     [TestCase("/arena/get_challenge_info")]
+    [TestCase("/arena/get_challenge_ranking_history")]
     public async Task Authenticated_route_resolves(string path)
     {
         using var factory = new TestFactory();