feat: add WebSocket protocol constants (RFC 6455)

Port WsConstants from golang/nats-server/server/websocket.go lines 41-106.
Includes opcodes, frame header bits, close status codes, compression
constants, header names, path routing, and the WsClientKind enum.

tests/NATS.Server.Tests/WebSocket/WsConstantsTests.cs

@@ -0,0 +1,53 @@
+using NATS.Server.WebSocket;
+using Shouldly;
+
+namespace NATS.Server.Tests.WebSocket;
+
+public class WsConstantsTests
+{
+    [Fact]
+    public void OpCodes_MatchRfc6455()
+    {
+        WsConstants.TextMessage.ShouldBe(1);
+        WsConstants.BinaryMessage.ShouldBe(2);
+        WsConstants.CloseMessage.ShouldBe(8);
+        WsConstants.PingMessage.ShouldBe(9);
+        WsConstants.PongMessage.ShouldBe(10);
+    }
+
+    [Fact]
+    public void FrameBits_MatchRfc6455()
+    {
+        WsConstants.FinalBit.ShouldBe((byte)0x80);
+        WsConstants.Rsv1Bit.ShouldBe((byte)0x40);
+        WsConstants.MaskBit.ShouldBe((byte)0x80);
+    }
+
+    [Fact]
+    public void CloseStatusCodes_MatchRfc6455()
+    {
+        WsConstants.CloseStatusNormalClosure.ShouldBe(1000);
+        WsConstants.CloseStatusGoingAway.ShouldBe(1001);
+        WsConstants.CloseStatusProtocolError.ShouldBe(1002);
+        WsConstants.CloseStatusPolicyViolation.ShouldBe(1008);
+        WsConstants.CloseStatusMessageTooBig.ShouldBe(1009);
+    }
+
+    [Theory]
+    [InlineData(WsConstants.CloseMessage)]
+    [InlineData(WsConstants.PingMessage)]
+    [InlineData(WsConstants.PongMessage)]
+    public void IsControlFrame_True(int opcode)
+    {
+        WsConstants.IsControlFrame(opcode).ShouldBeTrue();
+    }
+
+    [Theory]
+    [InlineData(WsConstants.TextMessage)]
+    [InlineData(WsConstants.BinaryMessage)]
+    [InlineData(0)]
+    public void IsControlFrame_False(int opcode)
+    {
+        WsConstants.IsControlFrame(opcode).ShouldBeFalse();
+    }
+}