fix(battle-node): preserve long type on numeric array elements in FromJson

Root cause for the lingering mulligan failure: the inline conditional
expression in MsgEnvelope.ToObject

    JsonValueKind.Number => el.TryGetInt64(out var l) ? l : el.GetDouble(),

unified its branches to the common implicit-convertible type. long→double
is implicit, so both branches collapsed to double and the integer value
silently widened. Inside an array (idxList:[2]), each element came back
as boxed double; OfType<long> in ExtractIdxList then filtered every
entry out, so swapIndices arrived empty and BuildSwapResponse echoed
the unchanged hand — exactly the diff-against-Deal mismatch the client
flagged as "Card swap failed: AbandonCards[2]/DrawCards[]".

Extract a ParseNumber helper that returns object explicitly so each
branch boxes its own runtime type. Also harden ExtractIdxList to accept
any boxed numeric type (long/int/double/decimal/string) so a future
JSON-parser drift can't silently regress this path again.

Two regression tests:
- FromJson_NumericArray_PreservesLongTypeOnEachElement: confirms the
  fix at the JSON-parse layer with a hardcoded "{\"idxList\":[2,3]}".
- Swap_WithIdxListContainingTwo_ProducesHandWithFreshIdxAtPosition1:
  exercises the dispatch end-to-end with a Body holding a real boxed
  long; asserts position 1 of the response hand is the fresh deck idx 4.

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

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -266,8 +266,26 @@ public sealed class BattleSession
 
     private static IReadOnlyList<long> ExtractIdxList(MsgEnvelope env)
     {
-        if (env.Body.TryGetValue("idxList", out var raw) && raw is List<object?> lst)
-            return lst.OfType<long>().ToList();
+        // Defensive: accept any IEnumerable carrying any numeric boxing (long/int/double/decimal/
+        // string). MsgEnvelope.FromJson should box small ints as long, but a parser quirk
+        // anywhere upstream could yield a different boxed type and OfType<long> would silently
+        // drop the entries — that broke the v1 mulligan during smoke.
+        if (env.Body.TryGetValue("idxList", out var raw) && raw is System.Collections.IEnumerable seq && raw is not string)
+        {
+            var result = new List<long>();
+            foreach (var item in seq)
+            {
+                switch (item)
+                {
+                    case long l: result.Add(l); break;
+                    case int i: result.Add(i); break;
+                    case double d: result.Add((long)d); break;
+                    case decimal m: result.Add((long)m); break;
+                    case string s when long.TryParse(s, out var p): result.Add(p); break;
+                }
+            }
+            return result;
+        }
         return Array.Empty<long>();
     }