Fix MQTT disconnection issue by adding KeepAlive configuration

- Add KeepAlivePeriodSeconds to MqttConfig (default: 60 seconds)
- Configure MQTT client with KeepAlive period in initial connection
- Configure MQTT client with KeepAlive period in reconnection logic
- Update appsettings.json with KeepAlivePeriodSeconds setting

Issue:
- MQTT broker was disconnecting the client after ~3 seconds of inactivity
- Client was not sending keepalive packets (PINGREQ) to maintain connection
- This caused repeated connect/disconnect cycles

Solution:
- Added WithKeepAlivePeriod() to MQTT client options
- Client now sends PINGREQ packets every 60 seconds (configurable)
- Prevents broker from disconnecting idle clients
- Standard MQTT keepalive behavior

The keepalive period can be configured via:
- appsettings.json: MQTT.KeepAlivePeriodSeconds
- Environment variable: MQTT__KeepAlivePeriodSeconds

Default: 60 seconds (standard MQTT keepalive interval)

Author: Alex J Lennon

src/AppConfig.cs

@@ -46,6 +46,7 @@ public class MqttConfig
     public double RetryBackoffMultiplier { get; set; } = 2.0;
     public bool AutoReconnect { get; set; } = true;
     public int ReconnectDelaySeconds { get; set; } = 5;
+    public int KeepAlivePeriodSeconds { get; set; } = 60;
     public bool UseTls { get; set; } = false;
     public bool AllowUntrustedCertificates { get; set; } = false;
     public string? CertificatePath { get; set; }

src/MQTTControl.cs

@@ -142,11 +142,16 @@ public static async Task Initialise(CancellationTokenSource cts, AppConfig? conf
             return Task.CompletedTask;
         };
 
+        // Get keepalive period from config (default: 60 seconds)
+        // KeepAlive prevents broker from disconnecting idle clients
+        int keepAlivePeriod = _config?.MQTT.KeepAlivePeriodSeconds ?? 60;
+        
         var builder = new MqttClientOptionsBuilder()
             .WithClientId(_clientId)
             .WithTcpServer(_serverAddress, _port)
             .WithCleanSession()
-            .WithTimeout(TimeSpan.FromSeconds(_timeoutInSeconds));
+            .WithTimeout(TimeSpan.FromSeconds(_timeoutInSeconds))
+            .WithKeepAlivePeriod(TimeSpan.FromSeconds(keepAlivePeriod));
 
         // Add TLS/SSL if configured
         if (_config?.MQTT.UseTls == true)
@@ -351,11 +356,15 @@ private static async Task AttemptReconnectAsync(CancellationToken cancellationTo
             return;
         }
 
+        // Get keepalive period from config (default: 60 seconds)
+        int keepAlivePeriod = _config?.MQTT.KeepAlivePeriodSeconds ?? 60;
+        
         var builder = new MqttClientOptionsBuilder()
             .WithClientId(_clientId)
             .WithTcpServer(_serverAddress, _port)
             .WithCleanSession()
-            .WithTimeout(TimeSpan.FromSeconds(_timeoutInSeconds));
+            .WithTimeout(TimeSpan.FromSeconds(_timeoutInSeconds))
+            .WithKeepAlivePeriod(TimeSpan.FromSeconds(keepAlivePeriod));
 
         // Add TLS/SSL if configured (same as initial connection)
         if (_config?.MQTT.UseTls == true)

src/appsettings.json

@@ -13,6 +13,7 @@
     "RetryBackoffMultiplier": 2.0,
     "AutoReconnect": true,
     "ReconnectDelaySeconds": 5,
+    "KeepAlivePeriodSeconds": 60,
     "UseTls": false,
     "AllowUntrustedCertificates": false,
     "CertificatePath": null,