feat: add per-subsystem log control via --log_level_override CLI flag

Adds LogOverrides property to NatsOptions and a --log_level_override=namespace=level CLI flag that wires Serilog MinimumLevel.Override entries so operators can tune verbosity per .NET namespace without changing the global log level.

src/NATS.Server.Host/Program.cs

@@ -85,6 +85,14 @@ for (int i = 0; i < args.Length; i++)
         case "--service":
             windowsService = true;
             break;
+        case "--log_level_override" when i + 1 < args.Length:
+            var parts = args[++i].Split('=', 2);
+            if (parts.Length == 2)
+            {
+                options.LogOverrides ??= new();
+                options.LogOverrides[parts[0]] = parts[1];
+            }
+            break;
     }
 }
 
@@ -135,6 +143,16 @@ else if (options.Syslog)
     logConfig.WriteTo.LocalSyslog("nats-server");
 }
 
+// Apply per-subsystem log level overrides
+if (options.LogOverrides is not null)
+{
+    foreach (var (ns, level) in options.LogOverrides)
+    {
+        if (Enum.TryParse<Serilog.Events.LogEventLevel>(level, true, out var serilogLevel))
+            logConfig.MinimumLevel.Override(ns, serilogLevel);
+    }
+}
+
 Log.Logger = logConfig.CreateLogger();
 
 if (windowsService)