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.UnitTests/BattleNode/Protocol/MsgEnvelopeTests.cs

@@ -7,6 +7,26 @@ namespace SVSim.UnitTests.BattleNode.Protocol;
 [TestFixture]
 public class MsgEnvelopeTests
 {
+    [Test]
+    public void FromJson_NumericArray_PreservesLongTypeOnEachElement()
+    {
+        // Regression for the v1 mulligan bug: idxList:[2] from the wire was decoded as a list
+        // containing a boxed double (2.0) instead of a boxed long (2L). The conditional expression
+        // `el.TryGetInt64(out var l) ? l : el.GetDouble()` unified its branches to the common
+        // implicit type (double) and silently widened the long. Downstream OfType<long> filtered
+        // every entry out, so swapIndices arrived empty and the server echoed the unchanged hand.
+        const string json = "{\"uri\":\"Swap\",\"viewerId\":1,\"uuid\":\"u\",\"try\":0,\"cat\":1,\"idxList\":[2,3]}";
+
+        var env = MsgEnvelope.FromJson(json);
+
+        var idxList = (List<object?>)env.Body["idxList"]!;
+        Assert.That(idxList.Count, Is.EqualTo(2));
+        Assert.That(idxList[0], Is.TypeOf<long>(), "idxList[0] must be boxed long, not double.");
+        Assert.That(idxList[0], Is.EqualTo(2L));
+        Assert.That(idxList[1], Is.TypeOf<long>());
+        Assert.That(idxList[1], Is.EqualTo(3L));
+    }
+
     [Test]
     public void Roundtrip_PreservesEnvelopeAndBody()
     {