feat(battle-node): EngineIoFrame parse/encode for EIO3 packets

SVSim.UnitTests/BattleNode/Wire/EngineIoFrameTests.cs

@@ -0,0 +1,51 @@
+using NUnit.Framework;
+using SVSim.BattleNode.Wire;
+
+namespace SVSim.UnitTests.BattleNode.Wire;
+
+[TestFixture]
+public class EngineIoFrameTests
+{
+    [TestCase("0{\"sid\":\"abc\"}", EngineIoPacketType.Open, "{\"sid\":\"abc\"}")]
+    [TestCase("2", EngineIoPacketType.Ping, "")]
+    [TestCase("3", EngineIoPacketType.Pong, "")]
+    [TestCase("4hello", EngineIoPacketType.Message, "hello")]
+    public void Parse_RecognizesTypeAndPayload(string raw, EngineIoPacketType expectedType, string expectedPayload)
+    {
+        var frame = EngineIoFrame.Parse(raw);
+
+        Assert.That(frame.Type, Is.EqualTo(expectedType));
+        Assert.That(frame.Payload, Is.EqualTo(expectedPayload));
+    }
+
+    [TestCase(EngineIoPacketType.Open, "{\"sid\":\"abc\"}", "0{\"sid\":\"abc\"}")]
+    [TestCase(EngineIoPacketType.Pong, "", "3")]
+    [TestCase(EngineIoPacketType.Message, "2[\"msg\",{}]", "42[\"msg\",{}]")]
+    public void Encode_EmitsTypeDigitFollowedByPayload(EngineIoPacketType type, string payload, string expected)
+    {
+        var frame = new EngineIoFrame(type, payload);
+
+        Assert.That(frame.Encode(), Is.EqualTo(expected));
+    }
+
+    [Test]
+    public void Parse_EmptyInput_Throws()
+    {
+        Assert.Throws<ArgumentException>(() => EngineIoFrame.Parse(""));
+    }
+
+    [Test]
+    public void Parse_NonDigitFirstChar_Throws()
+    {
+        Assert.Throws<ArgumentException>(() => EngineIoFrame.Parse("xfoo"));
+    }
+
+    [Test]
+    public void EngineIoHandshake_SerializesWithCamelCaseKeys()
+    {
+        var hs = new EngineIoHandshake("abc123", Array.Empty<string>(), 25000, 60000);
+
+        Assert.That(hs.ToJson(),
+            Is.EqualTo("{\"sid\":\"abc123\",\"upgrades\":[],\"pingInterval\":25000,\"pingTimeout\":60000}"));
+    }
+}