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

SVSim.BattleNode/Wire/EngineIoHandshake.cs

@@ -0,0 +1,22 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace SVSim.BattleNode.Wire;
+
+/// <summary>
+/// Payload of an EIO3 Open packet. Sent by the server to the client immediately after the WS upgrade.
+/// </summary>
+public sealed record EngineIoHandshake(
+    [property: JsonPropertyName("sid")] string Sid,
+    [property: JsonPropertyName("upgrades")] string[] Upgrades,
+    [property: JsonPropertyName("pingInterval")] int PingInterval,
+    [property: JsonPropertyName("pingTimeout")] int PingTimeout)
+{
+    // Wire-key casing here is bare camelCase — NOT EmulatedEntrypoint's snake_case policy.
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+    };
+
+    public string ToJson() => JsonSerializer.Serialize(this, Options);
+}