feat: enforce mqtt auth tls and keepalive semantics

tests/NATS.Server.Tests/Mqtt/MqttSessionRuntimeTests.cs

@@ -68,4 +68,22 @@ internal static class MqttRuntimeWire
 
         return Encoding.UTF8.GetString([.. bytes]);
     }
+
+    public static async Task<string?> ReadRawAsync(NetworkStream stream, int timeoutMs)
+    {
+        using var timeout = new CancellationTokenSource(timeoutMs);
+        var one = new byte[1];
+        try
+        {
+            var read = await stream.ReadAsync(one.AsMemory(0, 1), timeout.Token);
+            if (read == 0)
+                return null;
+
+            return Encoding.UTF8.GetString(one, 0, read);
+        }
+        catch (OperationCanceledException)
+        {
+            return "__timeout__";
+        }
+    }
 }