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.

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));
+    }
 }