fix(rank-battle): route ai-start through the queue-time MatchContext

Live-smoke bug 2026-06-02: queued Bloodcraft (deck #5), wire showed
classId=2 (Swordcraft) for self_info on the /ai_unlimited_rank_battle/start
response — client rendered the wrong leader.

Two layers of the same bug:

1. MatchContextBuilder.BuildForRankBattleAsync hardcoded deckNo=1 instead
   of taking it from the do_matching request — verified against
   data_dumps/captures/traffic.ndjson L17 where deck_no=5 was on the wire.
   Signature changes to (viewerId, format, deckNo); DoMatchingInternal
   passes req.DeckNo.

2. AiStartInternal rebuilt MatchContext from scratch — but the /ai_*/start
   request body is BaseRequest only, no deck_no on the wire. The fix uses
   the MatchContext the bridge already stored at do_matching resolution time
   (in the Bot PendingBattle), so deck/cosmetic data is consistent end-to-end.
   New IBattleSessionStore.TryFindPendingForViewer(viewerId) finds the
   viewer's pending battle for lookup. The store entry persists across
   ai_start (idempotent reads are fine — the WS handler removes on connect).
   No-pending sentinel: ai_id=-1 surfaces the "no AI assigned" error in the
   client.

Tests: 936 → 939 passing.
- MatchContextBuilderTests.BuildForRankBattle_uses_the_caller_supplied_deck_number
  seeds deck #1 (class 1) and deck #5 (class 6) and asserts the deckNo
  argument picks the right one.
- RankBattleControllerTests.AiStart_self_info_class_matches_queued_deck_number
  is the end-to-end regression: register Bot battle with deck #5, hit
  /ai_unlimited_rank_battle/start, assert self_info.classId == 6.
- RankBattleControllerTests.AiStart_without_pending_battle_returns_neg1_sentinel
  locks the defensive ai_id=-1 path.
- Existing AiStart_* tests bypass do_matching, so adapted to call a new
  RegisterBotBattleAsync helper that mirrors what InProcessPairUp does on
  AI-fallback resolution.

SeedDeckAsync gains an optional classId so test cases can differentiate
decks by class (was always picking Classes.First()).

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

SVSim.UnitTests/Infrastructure/SVSimTestFactory.cs

@@ -369,13 +369,16 @@ internal sealed class SVSimTestFactory : WebApplicationFactory<Program>
     /// path. Picks the first seeded class/sleeve/leader-skin from the master tables; tests
     /// that need specific ids should hit the DB directly.
     /// </summary>
-    public async Task SeedDeckAsync(long viewerId, Format format, int number, string name = "Test Deck")
+    public async Task SeedDeckAsync(long viewerId, Format format, int number, string name = "Test Deck", int? classId = null)
     {
         using var scope = Services.CreateScope();
         var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
         var repo = scope.ServiceProvider.GetRequiredService<IDeckRepository>();
 
-        var cls = await db.Classes.FirstAsync();
+        var cls = classId is null
+            ? await db.Classes.FirstAsync()
+            : await db.Classes.FindAsync(classId.Value)
+                ?? throw new InvalidOperationException($"SeedDeckAsync: class {classId} not found");
         var sleeve = await db.Sleeves.FirstAsync();
         var skin = await db.LeaderSkins.FirstAsync();