refactor: extract NATS.Server.Gateways.Tests project

Move 25 gateway-related test files from NATS.Server.Tests into a
dedicated NATS.Server.Gateways.Tests project. Update namespaces,
replace private ReadUntilAsync with SocketTestHelper from TestUtilities,
inline TestServerFactory usage, add InternalsVisibleTo, and register
the project in the solution file. All 261 tests pass.

tests/NATS.Server.Gateways.Tests/Gateways/GatewayCommandTests.cs

@@ -0,0 +1,99 @@
+using System.Text;
+using NATS.Server.Gateways;
+using Shouldly;
+
+namespace NATS.Server.Gateways.Tests.Gateways;
+
+// Go reference: gateway.go:90-120 — gateway protocol constants and command formatting.
+
+public class GatewayCommandTests
+{
+    [Fact]
+    public void FormatSub_produces_correct_wire_format()
+    {
+        // Go reference: gateway.go — RS+ propagation, sendGatewaySubsToGateway
+        var bytes = GatewayCommands.FormatSub("$G", "orders.>");
+        var line = Encoding.UTF8.GetString(bytes);
+        line.ShouldBe("GS+ $G orders.>\r\n");
+    }
+
+    [Fact]
+    public void FormatUnsub_produces_correct_wire_format()
+    {
+        // Go reference: gateway.go — RS- propagation, sendGatewayUnsubToGateway
+        var bytes = GatewayCommands.FormatUnsub("$G", "orders.>");
+        var line = Encoding.UTF8.GetString(bytes);
+        line.ShouldBe("GS- $G orders.>\r\n");
+    }
+
+    [Fact]
+    public void FormatMode_optimistic_produces_O()
+    {
+        // Go reference: gateway.go — GMODE command, "O" = Optimistic mode
+        var bytes = GatewayCommands.FormatMode("$G", GatewayInterestMode.Optimistic);
+        var line = Encoding.UTF8.GetString(bytes);
+        line.ShouldBe("GMODE $G O\r\n");
+    }
+
+    [Fact]
+    public void FormatMode_interest_only_produces_I()
+    {
+        // Go reference: gateway.go — GMODE command, "I" = InterestOnly mode
+        var bytes = GatewayCommands.FormatMode("$G", GatewayInterestMode.InterestOnly);
+        var line = Encoding.UTF8.GetString(bytes);
+        line.ShouldBe("GMODE $G I\r\n");
+    }
+
+    [Fact]
+    public void ParseCommandType_identifies_sub()
+    {
+        // Go reference: gateway.go — processGatewayMsg dispatch on GS+
+        var line = Encoding.UTF8.GetBytes("GS+ ACC foo.bar\r\n");
+        var result = GatewayCommands.ParseCommandType(line);
+        result.ShouldBe(GatewayCommandType.Sub);
+    }
+
+    [Fact]
+    public void ParseCommandType_identifies_unsub()
+    {
+        // Go reference: gateway.go — processGatewayMsg dispatch on GS-
+        var line = Encoding.UTF8.GetBytes("GS- ACC foo.bar\r\n");
+        var result = GatewayCommands.ParseCommandType(line);
+        result.ShouldBe(GatewayCommandType.Unsub);
+    }
+
+    [Fact]
+    public void ParseCommandType_identifies_ping()
+    {
+        // Go reference: gateway.go — keepalive GPING command
+        var result = GatewayCommands.ParseCommandType(GatewayCommands.Ping);
+        result.ShouldBe(GatewayCommandType.Ping);
+    }
+
+    [Fact]
+    public void ParseCommandType_identifies_pong()
+    {
+        // Go reference: gateway.go — keepalive GPONG response
+        var result = GatewayCommands.ParseCommandType(GatewayCommands.Pong);
+        result.ShouldBe(GatewayCommandType.Pong);
+    }
+
+    [Fact]
+    public void ParseCommandType_returns_null_for_unknown()
+    {
+        // Unrecognized commands should return null for graceful handling
+        var line = Encoding.UTF8.GetBytes("UNKNOWN something\r\n");
+        var result = GatewayCommands.ParseCommandType(line);
+        result.ShouldBeNull();
+    }
+
+    [Fact]
+    public void Wire_format_constants_end_correctly()
+    {
+        // Go reference: gateway.go — all gateway commands use CRLF line endings
+        var crlf = new byte[] { (byte)'\r', (byte)'\n' };
+        GatewayCommands.Ping.TakeLast(2).ShouldBe(crlf);
+        GatewayCommands.Pong.TakeLast(2).ShouldBe(crlf);
+        GatewayCommands.Crlf.ShouldBe(crlf);
+    }
+}