fix(battle-node): clip SIO ack arg instead of checked-cast throwing on overflow

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -342,10 +342,35 @@ public sealed class BattleSession
         }
     }
 
+    /// <summary>
+    /// Clip a long ack arg into the int range Socket.IO v2's typed AckResponse API accepts.
+    /// Logs a warning on clip; the implausibly-large pubSeq case is observationally
+    /// indistinguishable at the client (BestHTTP.SocketIO discards the echoed value), so
+    /// clipping is safer than the prior <c>checked((int)arg)</c> that threw and killed the
+    /// session on overflow.
+    /// </summary>
+    internal static int ClipAckArg(long arg, ILogger log, string battleId)
+    {
+        if (arg > int.MaxValue)
+        {
+            log.LogWarning(
+                "BattleSession {Bid}: pubSeq {Seq} exceeds int.MaxValue; clipping ack arg.",
+                battleId, arg);
+            return int.MaxValue;
+        }
+        if (arg < int.MinValue)
+        {
+            log.LogWarning(
+                "BattleSession {Bid}: pubSeq {Seq} below int.MinValue; clipping ack arg.",
+                battleId, arg);
+            return int.MinValue;
+        }
+        return (int)arg;
+    }
+
     private async Task SendSioAckAsync(int ackId, long arg)
     {
-        // checked() ensures we'd notice if pubSeq ever exceeded int.MaxValue (not realistic but defensive).
-        var ack = SocketIoFrame.AckResponse(ackId, checked((int)arg));
+        var ack = SocketIoFrame.AckResponse(ackId, ClipAckArg(arg, _log, BattleId));
         var (text, _) = ack.Encode();
         var eioText = $"{(int)EngineIoPacketType.Message}{text}";
         await SendTextAsync(eioText, _sessionCt);

SVSim.UnitTests/BattleNode/Sessions/ClipAckArgTests.cs

@@ -0,0 +1,44 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NUnit.Framework;
+using SVSim.BattleNode.Sessions;
+
+namespace SVSim.UnitTests.BattleNode.Sessions;
+
+[TestFixture]
+public class ClipAckArgTests
+{
+    [Test]
+    public void InRange_ReturnsArgUnchanged()
+    {
+        var result = BattleSession.ClipAckArg(42L, NullLogger.Instance, battleId: "b");
+        Assert.That(result, Is.EqualTo(42));
+    }
+
+    [Test]
+    public void AboveIntMax_ClipsToIntMaxValue()
+    {
+        var result = BattleSession.ClipAckArg((long)int.MaxValue + 1L, NullLogger.Instance, battleId: "b");
+        Assert.That(result, Is.EqualTo(int.MaxValue));
+    }
+
+    [Test]
+    public void BelowIntMin_ClipsToIntMinValue()
+    {
+        var result = BattleSession.ClipAckArg((long)int.MinValue - 1L, NullLogger.Instance, battleId: "b");
+        Assert.That(result, Is.EqualTo(int.MinValue));
+    }
+
+    [Test]
+    public void AtIntMaxBoundary_ReturnsIntMaxValue()
+    {
+        var result = BattleSession.ClipAckArg((long)int.MaxValue, NullLogger.Instance, battleId: "b");
+        Assert.That(result, Is.EqualTo(int.MaxValue));
+    }
+
+    [Test]
+    public void AtIntMinBoundary_ReturnsIntMinValue()
+    {
+        var result = BattleSession.ClipAckArg((long)int.MinValue, NullLogger.Instance, battleId: "b");
+        Assert.That(result, Is.EqualTo(int.MinValue));
+    }
+}