Fix ASP connection handler startup race

[suhashollakc]

Fixes #2173

Summary
This fixes a startup race in the ASP.NET Core connection handler.

If ASP.NET routes a connection to MqttConnectionHandler.OnConnectedAsync before the MQTT server adapter has been started, _serverOptions is still null and the handler throws a NullReferenceException. The connection should be terminated cleanly instead of remaining in an invalid handler state.

Changes

• Abort incoming ASP.NET Core connections when the MQTT server adapter has not been started or no client handler is wired.
• Read server options and the client handler into locals before constructing the MQTT connection context.
• Add a regression test covering the not-started handler path.

Validation

• dotnet test --project Source\MQTTnet.Tests\MQTTnet.Tests.csproj --framework net10.0 --no-restore --filter ConnectionHandler_Aborts_Connection_When_Server_Not_Started
• dotnet test --project Source\MQTTnet.Tests\MQTTnet.Tests.csproj --framework net10.0 --no-restore --filter FullyQualifiedName~MQTTnet.Tests.ASP
• dotnet build MQTTnet.slnx --no-restore

Note: the solution build completes successfully with existing benchmark warnings about obsolete ASP.NET hosting APIs.