fix: resolve slopwatch issues — add logging to empty catches and eliminate test timing delays

Replace empty catch blocks with meaningful log statements in NatsServer,
NatsClient, and Program. Add WaitForReadyAsync() to NatsServer for
deterministic server startup. Replace Task.Delay/Thread.Sleep in tests
with PING/PONG protocol flush and SubscribeCoreAsync for reliable
subscription synchronization.

src/NATS.Server/NatsServer.cs

@@ -15,11 +15,14 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly ServerInfo _serverInfo;
     private readonly ILogger<NatsServer> _logger;
     private readonly ILoggerFactory _loggerFactory;
+    private readonly TaskCompletionSource _listeningStarted = new(TaskCreationOptions.RunContinuationsAsynchronously);
     private Socket? _listener;
     private ulong _nextClientId;
 
     public SubList SubList => _subList;
 
+    public Task WaitForReadyAsync() => _listeningStarted.Task;
+
     public NatsServer(NatsOptions options, ILoggerFactory loggerFactory)
     {
         _options = options;
@@ -44,6 +47,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             _options.Host == "0.0.0.0" ? IPAddress.Any : IPAddress.Parse(_options.Host),
             _options.Port));
         _listener.Listen(128);
+        _listeningStarted.TrySetResult();
 
         _logger.LogInformation("Listening on {Host}:{Port}", _options.Host, _options.Port);
 
@@ -64,7 +68,10 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                 _ = RunClientAsync(client, ct);
             }
         }
-        catch (OperationCanceledException) { }
+        catch (OperationCanceledException)
+        {
+            _logger.LogDebug("Accept loop cancelled, server shutting down");
+        }
     }
 
     private async Task RunClientAsync(NatsClient client, CancellationToken ct)
@@ -73,9 +80,9 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         {
             await client.RunAsync(ct);
         }
-        catch (Exception)
+        catch (Exception ex)
         {
-            // Client disconnected or errored
+            _logger.LogDebug(ex, "Client {ClientId} disconnected with error", client.Id);
         }
         finally
         {