feat: add per-client trace delivery and echo control (Gap 5.7)

Implements ClientTraceInfo with TraceMsgDelivery recording and per-client
echo suppression; fixes AccountGoParityTests namespace ambiguity caused by
the new NATS.Server.Tests.Subscriptions test namespace.

src/NATS.Server/ClientTraceInfo.cs

@@ -0,0 +1,83 @@
+namespace NATS.Server;
+
+/// <summary>
+/// Per-client trace configuration and echo control.
+/// Go reference: server/client.go — c.trace, c.echo fields.
+/// </summary>
+public sealed class ClientTraceInfo
+{
+    private bool _traceEnabled;
+    private bool _echoEnabled = true; // default: echo is enabled
+    private readonly List<TraceRecord> _traceLog = new();
+    private readonly Lock _lock = new();
+
+    /// <summary>Whether message delivery tracing is enabled for this client.</summary>
+    public bool TraceEnabled
+    {
+        get => Volatile.Read(ref _traceEnabled);
+        set => Volatile.Write(ref _traceEnabled, value);
+    }
+
+    /// <summary>Whether this client echoes its own published messages back.</summary>
+    public bool EchoEnabled
+    {
+        get => Volatile.Read(ref _echoEnabled);
+        set => Volatile.Write(ref _echoEnabled, value);
+    }
+
+    /// <summary>
+    /// Records a message delivery trace if tracing is enabled.
+    /// Go reference: server/client.go — traceMsg / TraceMsgDelivery.
+    /// </summary>
+    public void TraceMsgDelivery(string subject, string destination, int payloadSize)
+    {
+        if (!TraceEnabled) return;
+        lock (_lock)
+        {
+            _traceLog.Add(new TraceRecord
+            {
+                Subject = subject,
+                Destination = destination,
+                PayloadSize = payloadSize,
+                TimestampUtc = DateTime.UtcNow,
+            });
+        }
+    }
+
+    /// <summary>
+    /// Returns whether a message from this client should be delivered back to it.
+    /// When echo is disabled, messages published by this client are not delivered to
+    /// subscriptions on the same client.
+    /// Go reference: server/client.go — c.echo check in deliverMsg.
+    /// </summary>
+    public bool ShouldEcho(string publisherClientId, string subscriberClientId)
+    {
+        if (EchoEnabled) return true;
+        return !string.Equals(publisherClientId, subscriberClientId, StringComparison.Ordinal);
+    }
+
+    /// <summary>Returns all trace records and clears the log.</summary>
+    public IReadOnlyList<TraceRecord> DrainTraceLog()
+    {
+        lock (_lock)
+        {
+            var copy = _traceLog.ToList();
+            _traceLog.Clear();
+            return copy;
+        }
+    }
+
+    /// <summary>Current trace log count.</summary>
+    public int TraceLogCount
+    {
+        get { lock (_lock) return _traceLog.Count; }
+    }
+}
+
+public sealed record TraceRecord
+{
+    public string Subject { get; init; } = string.Empty;
+    public string Destination { get; init; } = string.Empty;
+    public int PayloadSize { get; init; }
+    public DateTime TimestampUtc { get; init; }
+}