Add validations of variable byte integers (#2214)

* Add validations and add docs

* Update release notes

* Fix build

Source/MQTTnet/Exceptions/MqttProtocolViolationException.cs

@@ -6,4 +6,15 @@ using System;
 
 namespace MQTTnet.Exceptions;
 
-public class MqttProtocolViolationException(string message) : Exception(message);
+public class MqttProtocolViolationException(string message) : Exception(message)
+{
+    public const uint VariableByteIntegerMaxValue = 268435455;
+
+    public static void ThrowIfVariableByteIntegerExceedsLimit(uint value)
+    {
+        if (value > VariableByteIntegerMaxValue)
+        {
+            throw new MqttProtocolViolationException($"The value {value} is too large for a variable byte integer ({VariableByteIntegerMaxValue}).");
+        }
+    }
+}

Source/MQTTnet/Formatter/MqttBufferWriter.cs

@@ -19,7 +19,6 @@ namespace MQTTnet.Formatter;
 /// </summary>
 public sealed class MqttBufferWriter
 {
-    const uint VariableByteIntegerMaxValue = 268435455;
     const int EncodedStringMaxLength = 65535;
 
     readonly int _maxBufferSize;
@@ -223,10 +222,7 @@ public sealed class MqttBufferWriter
             return;
         }
 
-        if (value > VariableByteIntegerMaxValue)
-        {
-            throw new MqttProtocolViolationException($"The specified value ({value}) is too large for a variable byte integer.");
-        }
+        MqttProtocolViolationException.ThrowIfVariableByteIntegerExceedsLimit(value);
 
         var size = 0;
         var x = value;

Source/MQTTnet/MqttApplicationMessageBuilder.cs

@@ -21,7 +21,6 @@ public sealed class MqttApplicationMessageBuilder
     string _contentType;
     byte[] _correlationData;
     uint _messageExpiryInterval;
-
     MqttPayloadFormatIndicator _payloadFormatIndicator;
     ReadOnlySequence<byte> _payload;
     MqttQualityOfServiceLevel _qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce;
@@ -84,6 +83,8 @@ public sealed class MqttApplicationMessageBuilder
     /// </summary>
     public MqttApplicationMessageBuilder WithMessageExpiryInterval(uint messageExpiryInterval)
     {
+        // No validation required because this is a 4 byte integer!
+
         _messageExpiryInterval = messageExpiryInterval;
         return this;
     }
@@ -228,9 +229,11 @@ public sealed class MqttApplicationMessageBuilder
     /// </summary>
     public MqttApplicationMessageBuilder WithSubscriptionIdentifier(uint subscriptionIdentifier)
     {
+        MqttProtocolViolationException.ThrowIfVariableByteIntegerExceedsLimit(subscriptionIdentifier);
+
         if (_subscriptionIdentifiers == null)
         {
-            _subscriptionIdentifiers = new List<uint>();
+            _subscriptionIdentifiers = [];
         }
 
         _subscriptionIdentifiers.Add(subscriptionIdentifier);