feat(rank-battle): AiStart returns ai_id + camelCase self/oppo_info

AiStartInternal builds the self MatchContext, picks a bot from
IBotRoster, projects to the AiBattleStartResponseDto with camelCase
wire keys (sleeveId, emblemId, ... — see ai-start.md). turnState=0
(player first) is the safe default per the ai-start.md TODO; live
capture would clarify the enum.

No deck → ai_id=-1 fallback (the documented "no AI assigned" sentinel
per AIBattleStartTask.cs:21). 3 new wire-shape tests assert the
camelCase keys land verbatim in the JSON, plus self/oppo info come from
the right sources.

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

SVSim.EmulatedEntrypoint/Controllers/RankBattleController.cs

@@ -157,11 +157,66 @@ public sealed class RankBattleController : ControllerBase
         });
     }
 
-    // Filled in by Task 10.
-    private Task<IActionResult> AiStartInternal(Format format, CancellationToken ct)
+    private async Task<IActionResult> AiStartInternal(Format format, CancellationToken ct)
     {
-        if (!TryGetViewerId(out var _)) return Task.FromResult<IActionResult>(Unauthorized());
-        // Placeholder; real impl arrives in Task 10.
-        return Task.FromResult<IActionResult>(Ok(new AiBattleStartResponseDto { AiId = -1 }));
+        if (!TryGetViewerId(out var vid)) return Unauthorized();
+
+        MatchContext selfCtx;
+        try
+        {
+            selfCtx = await _ctxBuilder.BuildForRankBattleAsync(vid, format);
+        }
+        catch (InvalidOperationException ex)
+        {
+            // No deck → can't build a self profile. Surface as the "no AI assigned"
+            // sentinel; the client treats ai_id=-1 as a fallback/error condition.
+            _log.LogWarning(ex, "AiStart failed for viewer {Vid} format {Fmt}; returning ai_id=-1.", vid, format);
+            return Ok(new AiBattleStartResponseDto { AiId = -1 });
+        }
+
+        var bot = _botRoster.Pick(selfCtx);
+
+        // Per spec, ai-start.md TODO: turnState semantics unclear. Default 0 (player first).
+        return Ok(new AiBattleStartResponseDto
+        {
+            AiId = bot.AiId,
+            TurnState = 0,
+            SelfInfo = new AiBattlePlayerInfo
+            {
+                CountryCode = selfCtx.CountryCode,
+                UserName = selfCtx.UserName,
+                SleeveId = int.TryParse(selfCtx.SleeveId, out var sId) ? sId : -1,
+                EmblemId = int.TryParse(selfCtx.EmblemId, out var eId) ? eId : -1,
+                DegreeId = int.TryParse(selfCtx.DegreeId, out var dId) ? dId : -1,
+                FieldId = selfCtx.FieldId,
+                IsOfficial = selfCtx.IsOfficial,
+                OppoId = bot.AiId,
+                Seed = 0,
+                Rank = 0,
+                BattlePoint = 0,
+                ClassId = int.TryParse(selfCtx.ClassId, out var cId) ? cId : -1,
+                CharaId = int.TryParse(selfCtx.CharaId, out var chId) ? chId : -1,
+                IsMasterRank = 0,
+                MasterPoint = 0,
+            },
+            OppoInfo = new AiBattlePlayerInfo
+            {
+                CountryCode = bot.CountryCode,
+                UserName = bot.UserName,
+                SleeveId = bot.SleeveId,
+                EmblemId = bot.EmblemId,
+                DegreeId = bot.DegreeId,
+                FieldId = bot.FieldId,
+                IsOfficial = bot.IsOfficial,
+                OppoId = (int)vid,
+                Seed = 0,
+                Rank = bot.Rank,
+                BattlePoint = bot.BattlePoint,
+                ClassId = bot.ClassId,
+                CharaId = bot.CharaId,
+                IsMasterRank = bot.IsMasterRank,
+                MasterPoint = bot.MasterPoint,
+            },
+        });
     }
 }