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/Protocol/MsgEnvelope.cs

@@ -89,7 +89,14 @@ public sealed record MsgEnvelope(
     private static object? ToObject(JsonElement el) => el.ValueKind switch
     {
         JsonValueKind.String => el.GetString(),
-        JsonValueKind.Number => el.TryGetInt64(out var l) ? l : el.GetDouble(),
+        // Extracted to a helper because writing the conditional inline as
+        //   el.TryGetInt64(out var l) ? l : el.GetDouble()
+        // unifies the conditional's branches to the common implicit-convertible type. long→double
+        // is implicit; so the result type collapses to double and the long value silently widens.
+        // Downstream OfType<long> filters then drop the (now boxed-double) entries, which broke
+        // the mulligan idxList extraction. Separate method returns object explicitly so each
+        // branch boxes its own runtime type.
+        JsonValueKind.Number => ParseNumber(el),
         JsonValueKind.True => true,
         JsonValueKind.False => false,
         JsonValueKind.Null => null,
@@ -97,4 +104,10 @@ public sealed record MsgEnvelope(
         JsonValueKind.Object => el.EnumerateObject().ToDictionary(p => p.Name, p => ToObject(p.Value)),
         _ => el.GetRawText(),
     };
+
+    private static object ParseNumber(JsonElement el)
+    {
+        if (el.TryGetInt64(out var l)) return l;
+        return el.GetDouble();
+    }
 }