feat: add INatsClient interface and implement on NatsClient

Extract INatsClient interface from NatsClient to enable internal clients
(SYSTEM, ACCOUNT) to participate in the subscription system without
requiring a socket connection. Change Subscription.Client from concrete
NatsClient to INatsClient, keeping IMessageRouter and RemoveClient using
the concrete type since only socket clients need those paths.

src/NATS.Server/INatsClient.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Auth;
+using NATS.Server.Protocol;
+
+namespace NATS.Server;
+
+public interface INatsClient
+{
+    ulong Id { get; }
+    ClientKind Kind { get; }
+    bool IsInternal => Kind.IsInternal();
+    Account? Account { get; }
+    ClientOptions? ClientOpts { get; }
+    ClientPermissions? Permissions { get; }
+
+    void SendMessage(string subject, string sid, string? replyTo,
+        ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload);
+    bool QueueOutbound(ReadOnlyMemory<byte> data);
+    void RemoveSubscription(string sid);
+}

src/NATS.Server/NatsClient.cs

@@ -26,7 +26,7 @@ public interface ISubListAccess
     SubList SubList { get; }
 }
 
-public sealed class NatsClient : IDisposable
+public sealed class NatsClient : INatsClient, IDisposable
 {
     private readonly Socket _socket;
     private readonly Stream _stream;
@@ -45,6 +45,7 @@ public sealed class NatsClient : IDisposable
     private readonly ServerStats _serverStats;
 
     public ulong Id { get; }
+    public ClientKind Kind => ClientKind.Client;
     public ClientOptions? ClientOpts { get; private set; }
     public IMessageRouter? Router { get; set; }
     public Account? Account { get; private set; }

src/NATS.Server/Subscriptions/Subscription.cs

@@ -9,5 +9,5 @@ public sealed class Subscription
     public required string Sid { get; init; }
     public long MessageCount;  // Interlocked
     public long MaxMessages;   // 0 = unlimited
-    public NatsClient? Client { get; set; }
+    public INatsClient? Client { get; set; }
 }

tests/NATS.Server.Tests/InternalClientTests.cs

@@ -14,4 +14,16 @@ public class InternalClientTests
     {
         kind.IsInternal().ShouldBe(expected);
     }
+
+    [Fact]
+    public void NatsClient_implements_INatsClient()
+    {
+        typeof(NatsClient).GetInterfaces().ShouldContain(typeof(INatsClient));
+    }
+
+    [Fact]
+    public void NatsClient_kind_is_Client()
+    {
+        typeof(NatsClient).GetProperty("Kind")!.PropertyType.ShouldBe(typeof(ClientKind));
+    }
 }