feat: add InternalClient class for socketless internal messaging

src/NATS.Server/InternalClient.cs

@@ -0,0 +1,59 @@
+using NATS.Server.Auth;
+using NATS.Server.Protocol;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server;
+
+/// <summary>
+/// Lightweight socketless client for internal messaging (SYSTEM, ACCOUNT, JETSTREAM).
+/// Maps to Go's internal client created by createInternalClient() in server.go:1910-1936.
+/// No network I/O — messages are delivered via callback.
+/// </summary>
+public sealed class InternalClient : INatsClient
+{
+    public ulong Id { get; }
+    public ClientKind Kind { get; }
+    public bool IsInternal => Kind.IsInternal();
+    public Account? Account { get; }
+    public ClientOptions? ClientOpts => null;
+    public ClientPermissions? Permissions => null;
+
+    /// <summary>
+    /// Callback invoked when a message is delivered to this internal client.
+    /// Set by the event system or account import infrastructure.
+    /// </summary>
+    public Action<string, string, string?, ReadOnlyMemory<byte>, ReadOnlyMemory<byte>>? MessageCallback { get; set; }
+
+    private readonly Dictionary<string, Subscription> _subs = new(StringComparer.Ordinal);
+
+    public InternalClient(ulong id, ClientKind kind, Account account)
+    {
+        if (!kind.IsInternal())
+            throw new ArgumentException($"InternalClient requires an internal ClientKind, got {kind}", nameof(kind));
+
+        Id = id;
+        Kind = kind;
+        Account = account;
+    }
+
+    public void SendMessage(string subject, string sid, string? replyTo,
+        ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload)
+    {
+        MessageCallback?.Invoke(subject, sid, replyTo, headers, payload);
+    }
+
+    public bool QueueOutbound(ReadOnlyMemory<byte> data) => true; // no-op for internal clients
+
+    public void RemoveSubscription(string sid)
+    {
+        if (_subs.Remove(sid))
+            Account?.DecrementSubscriptions();
+    }
+
+    public void AddSubscription(Subscription sub)
+    {
+        _subs[sub.Sid] = sub;
+    }
+
+    public IReadOnlyDictionary<string, Subscription> Subscriptions => _subs;
+}