About the memory error problem of InjectApplicationMessage of MqttServer in client.WithCleanSession(false)

### Describe the bug
When I was doing performance optimization extension for MqttServer's InjectApplicationMessage , I used `ArrayPool` memory and unexpectedly found that the test [QoS_Tests.Preserve_Message_Order_For_Queued_Messages](https://github.com/dotnet/MQTTnet/blob/master/Source/MQTTnet.Tests/Server/QoS_Tests.cs#L115) failed because I cleared and recycled poolPayload after `await MqttServer.InjectApplicationMessage( poolPayload)` . I think there is no problem with my operation timing, and this behavior should be allowed.

If I use the byte[] poolPayload that I manually new, this test is successful. The reason is that the caller provides a payload in an independent space for InjectApplicationMessage, and this payload will only be used by MqttServer in the future until it is recycled by GC.

### Which component is your bug related to?

- Server

### To Reproduce
Modify the InjectApplicationMessage code of MqttServerExtensions and run the test Preserve_Message_Order_For_Queued_Messages.

```c#
public static async Task InjectApplicationMessage(
this MqttServer server,
string topic,
string payload = null,
MqttQualityOfServiceLevel qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce,
bool retain = false)
{
ArgumentNullException.ThrowIfNull(server);
ArgumentNullException.ThrowIfNull(topic);

var payloadBuffer = Encoding.UTF8.GetBytes(payload ?? string.Empty);

var message = new MqttApplicationMessageBuilder()
.WithTopic(topic)
.WithPayload(payloadBuffer)
.WithQualityOfServiceLevel(qualityOfServiceLevel)
.WithRetainFlag(retain)
.Build();

await server.InjectApplicationMessage(new InjectedMqttApplicationMessage(message));

// clear the buffer
payloadBuffer.AsSpan().Clear();
}
```
### Expected behavior
After await InjectApplicationMessage(), the memory reference to the payload must be released.