feat: add gateway command protocol with Go-compatible wire format (Gap 11.6)

Add GatewayCommands static class with wire-format byte sequences (GINFO, GS+, GS-,
GMODE, GMSG, GPING, GPONG) and FormatSub/FormatUnsub/FormatMode/ParseCommandType
helpers matching Go's gateway.go protocol constants. Add GatewayCommandType enum.
10 tests covering all wire formats and command parsing.

src/NATS.Server/Gateways/GatewayCommands.cs

@@ -0,0 +1,87 @@
+using System.Text;
+
+namespace NATS.Server.Gateways;
+
+/// <summary>
+/// Gateway wire protocol command types matching Go's gateway.go format.
+/// Go reference: gateway.go:90-120 — gateway protocol constants.
+/// </summary>
+public enum GatewayCommandType
+{
+    /// <summary>Gateway connection info exchange.</summary>
+    Info,
+    /// <summary>Subscribe interest notification.</summary>
+    Sub,
+    /// <summary>Unsubscribe interest notification.</summary>
+    Unsub,
+    /// <summary>Account interest mode change.</summary>
+    Mode,
+    /// <summary>Routed message through gateway.</summary>
+    Msg,
+    /// <summary>Ping keepalive.</summary>
+    Ping,
+    /// <summary>Pong keepalive response.</summary>
+    Pong,
+}
+
+/// <summary>
+/// Gateway wire protocol byte sequences and formatting helpers matching Go's gateway protocol.
+/// Go reference: gateway.go:90-120 — gateway protocol constants and command formatting.
+/// </summary>
+public static class GatewayCommands
+{
+    // Wire format byte sequences matching Go's gateway protocol
+    public static readonly byte[] InfoPrefix = "GINFO "u8.ToArray();
+    public static readonly byte[] SubPrefix = "GS+ "u8.ToArray();
+    public static readonly byte[] UnsubPrefix = "GS- "u8.ToArray();
+    public static readonly byte[] ModePrefix = "GMODE "u8.ToArray();
+    public static readonly byte[] MsgPrefix = "GMSG "u8.ToArray();
+    public static readonly byte[] Ping = "GPING\r\n"u8.ToArray();
+    public static readonly byte[] Pong = "GPONG\r\n"u8.ToArray();
+    public static readonly byte[] Crlf = "\r\n"u8.ToArray();
+
+    /// <summary>
+    /// Formats a gateway subscribe command.
+    /// Wire format: GS+ {account} {subject}\r\n
+    /// Go reference: gateway.go — sendGatewaySubsToGateway, RS+ propagation.
+    /// </summary>
+    public static byte[] FormatSub(string account, string subject)
+        => Encoding.UTF8.GetBytes($"GS+ {account} {subject}\r\n");
+
+    /// <summary>
+    /// Formats a gateway unsubscribe command.
+    /// Wire format: GS- {account} {subject}\r\n
+    /// Go reference: gateway.go — sendGatewayUnsubToGateway, RS- propagation.
+    /// </summary>
+    public static byte[] FormatUnsub(string account, string subject)
+        => Encoding.UTF8.GetBytes($"GS- {account} {subject}\r\n");
+
+    /// <summary>
+    /// Formats a gateway mode change command.
+    /// Wire format: GMODE {account} {mode}\r\n
+    /// Mode: "O" for Optimistic (send everything), "I" for Interest-only.
+    /// Go reference: gateway.go — switchAccountToInterestMode, GMODE command.
+    /// </summary>
+    public static byte[] FormatMode(string account, GatewayInterestMode mode)
+    {
+        var modeStr = mode == GatewayInterestMode.InterestOnly ? "I" : "O";
+        return Encoding.UTF8.GetBytes($"GMODE {account} {modeStr}\r\n");
+    }
+
+    /// <summary>
+    /// Parses a gateway command type from the first bytes of a line.
+    /// Returns null if the command prefix is unrecognized.
+    /// Go reference: gateway.go — processGatewayMsg command dispatch.
+    /// </summary>
+    public static GatewayCommandType? ParseCommandType(ReadOnlySpan<byte> line)
+    {
+        if (line.StartsWith(InfoPrefix)) return GatewayCommandType.Info;
+        if (line.StartsWith(SubPrefix)) return GatewayCommandType.Sub;
+        if (line.StartsWith(UnsubPrefix)) return GatewayCommandType.Unsub;
+        if (line.StartsWith(ModePrefix)) return GatewayCommandType.Mode;
+        if (line.StartsWith(MsgPrefix)) return GatewayCommandType.Msg;
+        if (line.StartsWith(Ping)) return GatewayCommandType.Ping;
+        if (line.StartsWith(Pong)) return GatewayCommandType.Pong;
+        return null;
+    }
+}